CORRELATION AND CONTINUITY.
 
 SEW-8TBKKT 6QUAB1
 
 Ex Lftria N 
 C. K. OGDEN 
 
 THE CORKELATION 
 
 OF 
 
 PHYSICAL FORCES 
 
 BY 
 
 W. E. GROVE, Q.C., M.A., F.R.S. 
 
 CORRESPONDING MEMBER OF THE ACADEMIES OF 
 ROME, TURIX, ETC. 
 
 FIFTH EDITION. 
 
 FOLLOWED BY A DISCOURSE OH 
 
 CONTINUITY 
 
 DELIVERED BY THE AUTHOR AS 
 PRESIDENT OF THE BRITISH ASSOCIATION AT NOTTINGHAM, MDCCCLXVI. 
 
 LONDON: 
 LONGMANS, GREEN, AND CO. 
 
 1867.
 
 T TT>T> 
 
 T7NTVT -- 
 
 PREFACE. 
 
 THE phrase ' Correlation of Physical Forces ' in 
 the sense in which I have used it, having 
 become recognised by a large number of scientific 
 writers, it would produce confusion were I now to 
 adopt another title. It would, perhaps, have been 
 better if I had in the first instance used the term 
 Co-relation, as the words ' correlate,' ' correlative,' 
 had acquired a peculiar metaphysical sense some- 
 what differing from that which I attached to the 
 substantive correlation. The passage in the text 
 (p. 247) explains the meaning I have given to the 
 term. 
 
 Twenty-five years having elapsed since I pro- 
 mulgated the views contained in this Essay, winch 
 were first advanced in a lecture at the London 
 Institution in January 1842, printed by the pro- 
 prietors, and subsequently more fully developed in 
 a course of lectures in 1843, published in abstract 
 in the * Literary Gazette,' I think it advisable
 
 v j PREFACE. 
 
 to add a little to the Preface with reference to 
 other labourers in the same field. 
 
 It has happened with this subject as with many 
 others, that similar ideas have independently pre- 
 sented themselves to different minds about the same 
 period. In ' Leibig and Wohler's Annalen' for 
 May 1842,* a paper appeared by M. Mayer which 
 I had not read when my third edition was published, 
 but which I have now read in the translation by 
 Mr. Youmans of New York. It deduces very much 
 the same conclusions to which I had been led, 
 the author starting partly from h priori reasoning 
 and partly from an experiment by which water 
 was heated by agitation, and from another, which 
 had, however, previously been made by Davy, viz. 
 that ice can be melted by friction, though kept 
 in a medium which is below the freezing point of 
 water. 
 
 In 1843 a paper by Mr. Joule on the mechanical 
 equivalent of heat appeared, which, though not 
 in terms touching on the mutual and necessary 
 dependence of all the Physical Forces, yet bears 
 most importantly upon the doctrine. 
 
 While my third edition was going through the 
 press I had the good fortune to make the acquaint- 
 
 * I am informed that these papers are, in fact, published some 
 time f\fter the date at which they appear in the magazine.
 
 PREFACE. Vll 
 
 ance of M. Seguin, who informed me that his uncle, 
 the eminent Montgolfier, had long entertained the 
 idea that force was indestructible, though, with 
 the exception of one sentence in his paper on the 
 hydraulic ram, and where he is apparently speaking 
 of mechanical force, he has left nothing in print on 
 the subject. Not so, however, M. Seguin himself, 
 who in 1839, in a work on the ' Influence of Rail- 
 roads,' has distinctly expressed his uncle's and his 
 own views on the identity of heat and mechanical 
 force, and has given a calculation of their equivalent 
 relation, which is not far from the more recent 
 numerical results of Mayer, Joule, and others. 
 
 Several of the great mathematicians of a much 
 earlier period " advocated the idea of what they 
 termed the Conservation of Force, but although they 
 considered that a body in motion would so continue 
 for ever, unless arrested by the impact of another 
 body, and, indeed, in the latter case, would, if elastic, 
 still continue to move (though deflected from its 
 course) with a force proportionate to its elasticity, 
 yet with inelastic bodies the general and, as far as 
 I am aware, the universal belief was, that the motion 
 was arrested and the force annihilated. Montgolfier 
 went a step farther, and his hydraulic ram was to 
 him a proof of the truth of his preconceived idea,
 
 yiii PREFACE. 
 
 that the shock or impact of bodies left the me- 
 chanical force undestroyed.* 
 
 Previously, however, to the discoveries of the 
 voltaic battery, electro-magnetism, thermo-elec- 
 tricity, and photography, it was impossible for any 
 mind to perceive what, in the greater number of 
 cases, became of the force which was apparently lost. 
 The phenomena of heat, known from the earliest 
 tunes, would have been a mode of accounting for 
 the resulting force in many cases where motion was 
 arrested, and we find Bacon enouncing a theory that 
 motion was the form, as he quaintly termed it, of 
 heat. Rumford and Davy adopted this view, the 
 former with a fair approximative attempt at nu- 
 merical calculation, but no one of these philosophers 
 seems to have connected it with the indestructibility 
 of force. A passage in the writings of Dr. Roget, 
 combating the theory that mere contact of dis- 
 similar bodies was the source of voltaic electricity, 
 philosophically supports his argument by the idea 
 of non-creation of force. 
 
 As I have introduced into the later editions of my 
 Essay abstracts of the different discoveries which 
 I have found, since my first lectures, to bear upon 
 
 * See also a paper by Mr. Kankine, ' On the Dynamical Prin- 
 ciples of Newton,' in the 'Engineer,' Oct. 26, 1866.
 
 PREFACE. IX 
 
 the subject, I have been regarded by many rather as 
 the historian of the progress made in this branch 
 of thought than as one who has had anything to do 
 with its initiation. Everyone is but a poor judge 
 where he is himself interested, and I therefore write 
 with diffidence, but it would be affecting an indiffer- 
 ence which I do not feel if I did not state that I 
 believe myself to have been the first who introduced 
 this subject as a generalised system of philosophy, 
 and continued to enforce it in my lectures and 
 writings for many years, during which it met with 
 the opposition usual and proper to novel ideas. 
 
 Avocations necessary to the well-being of others 
 have prevented my following it up experimentally, 
 to the extent that I once hoped ; but I trust and 
 believe that this Essay, imperfect though it be, has 
 helped materially to impress on that portion of the 
 public which devotes its attention rather to the 
 philosophy of science than to what is now termed 
 science, the truth of the thesis advocated. 
 
 To show that the work of to-day is not sub- 
 stantially different from the thoughts I first pub- 
 lished on the subject, at a period when I knew 
 little or nothing of what had been thought before, 
 I venture to give a few extracts from the printed 
 copy of my lecture of 1842 :
 
 X PREFACE. 
 
 Physical Science treats of Matter, and what I shall to-night 
 term its Affections ; namely, Attraction, Motion, Heat, Light, 
 Electricity, Magnetism, Chemical- Affinity. When these re-act 
 upon matter, they constitute Forces. The present tendency of 
 theory seems to lead to the opinion that all these Affections are 
 resolvable into one, namely, Motion : however, should the 
 theories on these subjects be ultimately so effectually gene- 
 ralised as to become laws, they cannot avoid the necessity 
 for retaining different names for these different Affections ; or, 
 as they would then be called, different Modes of Motion. 
 
 (Ersted proved that Electricity and Magnetism are two forces 
 which act upon each other ; not in straight lines, as all other 
 known forces do, but in a rectangular direction : that i?, that 
 bodies invested with electricity, or the conduits of an electric- 
 current, tend to place magnets at right angles to them ; and, con- 
 versely, that magnets tend to place bodies conducting electricity 
 at right angles to them 
 
 The discovery of CErsted, by which electricity was made a 
 source of Magnetism, soon led philosophers to seek the converse 
 effect ; that is, to educe Electricity from a permanent magnet: 
 had these experimentalists succeeded in their expectations of 
 making a stationary magnet a source of electric-currents, they 
 would have realised the ancient dreams of perpetual motion, they 
 would have converted statics into dynamics, they would have 
 produced power without expenditure; in other words, they 
 would have become creators. They failed, and Faraday saw 
 their error : he proved that to obtain Electricity from Magnetism 
 it was necessary to superadd to this latter, motion ; that magneto 
 while in motion induced electricity in contiguous conductors ; and 
 that the direction of such electric-currents was tangential to the 
 polar direction of the magnet ; that as Dynamic-electricity may 
 be made the source of Magnetism and Motion, so Magnetism con- 
 joined with Motion may be made the source of Electricity. Here
 
 PREFACE. XI 
 
 originates the Science of Magneto-electricity, the true converse 
 of Electro-magnetism ; and thus between Electricity and Magne- 
 tism is shewn to exist a reciprocity of force such that, considering 
 either as the primary agent, the other becomes the re-agent ; 
 viewing one in the relation of cause, the other is the effect. 
 
 The Science of Thermo-Electricity connected heat with elec- 
 tricity, and proved these, like all other natural forces, to be 
 capable of mutual reaction 
 
 Voltaic action is Chemical action taking place at a distance, or 
 transferred through a chain of media : and the Daltonian equiva- 
 lent numbers are the exponents of the amount of voltaic action for 
 corresponding chemical substances. .... 
 
 By regarding the quantity of electrical, as directly proportional 
 to the efficient chemical action, and by experimentally tracing 
 this principle, I have been fortunate enough to increase the power 
 of the Voltaic-pile more than sixteen times, as compared with any 
 combination previously known 
 
 I am strongly disposed to consider that the facts of Catalysis 
 depend upon voltaic action, to generate which three heteroge- 
 neous substances are always necessary : Induced by this belief I 
 made some experiments on the subject, and succeeded in forming 
 a voltaic combination by gaseous-oxygen, gaseous-hydrogen, and 
 platinum ; by which a galvanometer was deflected and water 
 decomposed. . . . . 
 
 It appears to me that heat and light may be considered as 
 affections ; or, according to the Undulatory-theory, vibrations of 
 matter itself, and not of a distinct ethereal fluid permeating it : 
 these vibrations would be propagated, just as sound is propagated 
 by vibrations of wood or as waves by water. To my mind, all 
 the consequences of the Undulatory-theory flow as easily from 
 this, as from the hypothesis of a specific ether; to suppose 
 which, namely, to suppose a fluid sui generis, and of extreme 
 tenuity penetrating solid bodies, we must assume, first, the
 
 Xll PREFACE. 
 
 existence of the fluid itself; secondly, that bodies arc without 
 exception porous ; thirdly, that these pores communicate ; 
 fourthly, that matter is limited in expansibility. None of these 
 difficulties apply to the modification of this theory which I ven- 
 ture to propose ; and no other difficulty applies to it which does 
 not equally apply to the received hypothesis. With regard to 
 the planetary spaces, the diminishing periods of comets is a 
 strong argument for the existence of an universally-diffused 
 matter : this has the function of resistance, and there appean to 
 be no reason to divest it of the functions common to all matter, 
 or specifically to appropriate it to certain affections. Again, the 
 phenomena of transparency and opacity are, to my mind, more 
 easily explicable by the former than by the latter theory; as 
 resulting from a difference in the molecular .arrangement of the 
 matter affected. In regard to the effects of double-refraction 
 and polarisation, the molecular structure gives at once a reason 
 for the effects upon the one theory, while upon the other we 
 must, in addition to previous assumptions, further assume a 
 different elasticity of the ether in different directions within the 
 doubly-refracting medium. The same theory is applicable to 
 Electricity and Magnetism ; my own experiments on the influence 
 of the elastic intermedium on the voltaic-arc, and those of Faraday 
 on electrical induction, furnish strong arguments in support of 
 it. My inclination would lead me to detain you on this subject 
 much longer than my judgment deems advisable : I therefore 
 content myself with offering it to your consideration, and should 
 my avocations permit, I may at a future period more fully deve- 
 
 lopeit 
 
 Light, Heat, Electricity, Magnetism, Motion, and Chemical- 
 affinity, are all convertible material affections ; assuming- either 
 as the cause, one of the others will be the effect : thus heat may 
 be said to produce electricity, electricity to produce heat ; mag- 
 netism to produce electricity, electricity magnetism ; and so of 
 the rest. Cause and effect, therefore, in thc-ir abstract relation to
 
 PREFACE. Xlll 
 
 these forces, are words solely of convenience : we are totally un- 
 acquainted with the ultimate generating power of each and all of 
 them, and probably shall ever remain so ; we can only ascertain 
 the normas of their action : we must humbly refer their causation 
 to one omnipresent influence, and content ourselves with studying 
 their effects and developing by experiment their mutual relations. 
 
 I have transposed the passages relating to voltaic 
 action and catalysis, but I have not added a word 
 to the above quotations, and, as far as I am now 
 aware, the theory that the so-called imponderables 
 are affections of ordinary matter, that they are 
 resolvable into motion, that they are to be regarded 
 in their action on matter as forces, and not as 
 specific entities, and that they are capable of mutual 
 reaction, thence alternately acting as cause and 
 effect, had not at that time been publicly advanced.* 
 
 My original Essay being a record of lectures, and 
 being published by the managers of the Institution, 
 I necessarily adhered to the form and matter which 
 I had orally communicated. In preparing sub- 
 sequent editions I found that, without destroying 
 the identity of the work, I could not alter the style ; 
 although it would have been less difficult and more 
 satisfactory to me to have done so, the work would 
 not then have been a republication ; and I was for 
 
 * See also an experiment shown at the London Institution, 
 p. 297 post.
 
 xiv PREFACE. 
 
 obvious reasons anxious to preserve as far as I 
 could the original text, which, though added to, is 
 but little altered. 
 
 The form of lectures has necessarily continued 
 the use of the first person, and I would beg my 
 readers not to attribute to me, from the "modes 
 of expression used, a dogmatism which is far from 
 my thought. If my opinions are expressed broadly, 
 the reason is that when opinions are always hedged 
 in by qualifications, the style becomes embarrassed 
 and the meaning frequently unintelligible. 
 
 As the main object of a course of lectures is to 
 induce the auditor to think, and to consult works 
 on the subject he hears treated, so the object of 
 this Essay is more to induce a particular train of 
 thought on the known facts of physical science 
 than to enter with minute criticism into each 
 separate branch. 
 
 In one or two of the reviews of previous editions 
 the general idea of the work was objected to. J 
 believe, however, that will not now be the case ; 
 the mathematical labours of Mr. Thompson, 
 Clausius, and others, though not suitable for 
 insertion in an Essay such as tin's, have awakened 
 an interest for many portions of the subject, which 
 promises much for its future progress.
 
 PREFACE. XV 
 
 The short and irregular intervals which my pro- 
 fession permits me to devote to science so prevent 
 the continuity of attention necessary for the proper 
 evolution of a train of thought, that I certainly 
 should not now have courage to publish for the first 
 time such an Essay; and it is only the favour it 
 has received from those whose opinions I highly 
 value, and the, I trust pardonable, wish not to let 
 some favourite thoughts of my youth lose all con- 
 nection with my name, that have induced me to 
 reprint it. 
 
 My scientific readers will, I hope, excuse the 
 very short notices of certain branches of science 
 which are introduced, as without them the work 
 would be unintelligible to many for whom it is 
 intended. I have endeavoured so to arrange the 
 subjects that each division should form an intro- 
 duction to those which follow, and to assume no 
 more preliminary knowledge to be possessed by 
 my readers than would be expected from persons 
 acquainted with the elements of physical science. 
 
 The notes contain references to the original 
 memoirs in which the branches of science alluded 
 to are to be found, as well as to those which bear 
 on the main arguments ; where these memoirs are 
 numerous, or not easy of access, I have referred to 
 a
 
 Xvi PREFACE. 
 
 treatises in which they are collated. To prevent 
 the reader's attention being interrupted, I have in 
 the notes referred to the pages of the text, instead 
 of to interpolated letters. 
 
 For this fifth edition I have revised the whole 
 of the text, and several correspondents having 
 suggested that I should add to it my address as 
 President of the British Association, 1866, it 
 seemed to me on consideration not inappropriate. 
 In it are noticed several new discoveries relating 
 to the subjects treated of in this Essay, which, 
 had I not decided on adding the discourse, I should 
 have incorporated with the text. The doctrine 
 of continuity, however, though closely allied with 
 that of correlation, could not well have been inter- 
 woven with the Essay, and is more suitable as a 
 sequel. 
 
 I cannot but feel gratified with the reception 
 that address has met with, and although the por- 
 tions referring to the continuity of succession of 
 organised beings have, as I rather expected, been 
 the subject of some adverse comment, yet as I 
 have seen no argument against them, I have 
 nothing at present to answer or to alter. 
 
 Several of the arguments adduced by me were 
 written some years ago, and before the appearance
 
 PREFACE. XV11 
 
 of Darwin's own celebrated work. I had then 
 no notion of the effects of natural selection in 
 modifying organisms, but in order to test as fairly 
 as I could the reasons for and against continuity, 
 as opposed to special creations, I wrote down at 
 different times in the form of a dialogue every- 
 thing that occurred to me as bearing most strongly 
 on each side of the question, and showed it to 
 several friends with whom I discussed the subject. 
 My resulting opinion is given in the text. 
 
 I thought I had sufficiently guarded myself, in 
 the passage at p. 334, from being supposed to deny 
 that there are or have been catastrophes or 
 cataclysms; but it appears from some comments 
 that I have not done so. If sea or river under- 
 mine a cliff and the cliff fall, it is undoubtedly a 
 cataclysm ; if I tread on a beetle, it is a catas- 
 trophe to the beetle ; but formation and destruc- 
 tion are very different things, and the tenour 
 of my discourse applies to genesis not extinction. 
 Even in phenomena such as those of Geology, 
 though there were doubtless cataclysms, and some- 
 times on a much larger scale than at other times, 
 yet the evidence seems to me to point to their 
 being limited in extent at any one period, when 
 compared with the whole terrestrial surface.
 
 CORRELATION 
 
 PHYSICAL FORCES. 
 
 WHEN natural phenomena are for the first time 
 observed, a tendency immediately developes 
 itself to refer the.m to something previously known 
 to bring them within the range of acknowledged 
 sequences. The mode of regarding new facts, 
 which is most favourably received by the public, 
 is that which refers them to recognised views 
 stamps them into the mould in which the mind 
 has been already shaped. The new fact may be 
 far removed from those to which it is referred, 
 and may belong to a different order of analogies, 
 but this cannot then be known, as its co-ordinates 
 are wanting. It may be questionable whether the 
 mind is not so moulded by past events that it is 
 impossible to advance an entirely new view, but ad- 
 mitting such possibility, the new view, necessarily 
 B
 
 2 CORRELATION OF PHYSICAL FORCES. 
 
 founded on insufficient data, is likely to be more 
 incorrect and prejudicial than even a strained 
 attempt to reconcile the new discovery with known 
 facts. 
 
 The theory consequent upon new facts, whether 
 it be a co-ordination of them with known ones, or 
 the more difficult and dangerous attempt at re- 
 modelling the public ideas, is generally enunciated 
 by the discoverers themselves of the facts, or by 
 those to whose authority the world at the period 
 of the discovery defers ; others are not bold enough, 
 or if they be so, are unheeded. The earliest theories 
 thus enunciated obtain the firmest hold upon the 
 public mind, for at such a time there is no power of 
 testing, by a sufficient range of experience, the truth 
 of the theory ; it is accepted solely or mainly upon 
 authority: there being no means of contradiction, 
 its reception is, in the first instance, attended with 
 some degree of doubt, but as the time in which it 
 can fairly be investigated far exceeds that of any 
 lives then in being, and as neither the individual 
 nor the public mind will long tolerate a state of 
 abeyance, a theory shortly becomes, for want of 
 a better, admitted as an established truth: it is 
 handed from father to son, and gradually takes its 
 place in education. Succeeding generations, whose 
 minds are thus formed to an established view, are 
 much less likely to abandon it. They have adopted 
 it in the first instance, upon authority, to them
 
 INTRODUCTORY REMARKS. 3 
 
 unquestionable, and subsequently to yield up their 
 faith would involve a laborious remodelling of 
 ideas, a task which the public as a body will and 
 can rarely undertake, the frequent occurrence of 
 which is indeed inconsistent with the very exist- 
 ence of man in a social state, as it would induce 
 an anarchy of thought a perpetuity of mental 
 revolutions. 
 
 This necessity has its good ; but the prejudicial 
 effect upon the advance of science is, that by 
 this means, theories the most immature frequently 
 become the most permanent ; for no theory can be 
 more immature, none is likely to be so incorrect, as 
 that which is formed at the first flush of a new 
 discovery ; and though time exalts the authority of 
 those from whom it emanated, time can never give- 
 to the illustrious dead the means of analysing and 
 correcting erroneous views which subsequent dis- 
 coveries confer. 
 
 Take for instance the Ptolemaic System, which 
 we may almost literally explain by the expression 
 of Shakspeare : ' He that is giddy thinks the world 
 turns round.' We now see the error of this sys- 
 tem, because we have all an immediate opportunity 
 of refuting it ; but this identical error was received 
 as a truth for centuries, because, when first pro- 
 mulgated, the means of refuting it were not at 
 hand, and when the means of its refutation became 
 attainable, mankind had been so educated to the 
 
 B 2
 
 4 CORRELATION OF PHYSICAL FORCES. 
 
 supposed truth, that they rejected the proof of its 
 fallacy. 
 
 I have premised the above for two reasons : first, 
 to obtain a fair hearing, by requesting as far as 
 possible a dismissal from the minds of my readers 
 of preconceived views by and in favour of which 
 all are liable to be prejudiced ; and secondly, to 
 defend myself from the charge of undervaluing 
 authority, or treating lightly the opinions of those 
 to whom and to whose memory mankind looks 
 with reverence. Properly to value authority we 
 should estimate it together with its means of in- 
 formation : if ' a dwarf on the shoulders of a giant 
 can see further than the giant,' he is no less a 
 dwarf in comparison with the giant. 
 
 The subject on which I am about to treat viz., 
 the relation of the affections of matter to each 
 other and to matter peculiarly demands an un- 
 prejudiced regard. The different aspects under 
 which these agencies have been contemplated ; the 
 different views which have been taken of matter 
 itself; the metaphysical subtleties to which these 
 views unavoidably lead, if pursued beyond fair in- 
 ductions from existing experience, present diffi- 
 culties almost insurmountable. 
 
 The extent of claim which my views on this 
 subject may have to originality have been stated 
 in the Preface ; they became strongly impressed 
 upon my mind at a period when I was much
 
 INTRODUCTORY REMARKS. ^ 5 
 
 engaged in experimental research, and were, as 
 I then believed, and still believe, regarding them 
 as a system, new : expressions in the works of dif- 
 ferent authors, bearing more or less on the subject, 
 have subsequently been pointed out to me, some 
 of which go back to a distant period. An attempt 
 to analyse these in detail, and to trace how far I 
 have been anticipated by others, would probably 
 but little interest the reader, and in the course of 
 it I should constantly have to make distinctions 
 showing wherein I differed, and wherein I agreed 
 with others. I might cite authorities which appear 
 to me to oppose, and others which appear to co- 
 incide with certain of the views I have put forth ; 
 but this would interrupt the consecutive develope- 
 ment of my own ideas, and might render me liable 
 to the charge of misconstruing those of others; 
 I therefore think it better to avoid such discussion 
 in the text; and in addition to the sketch given in 
 the Preface, to furnish in the notes at the conclu- 
 sion such references to different authors as bear 
 upon the subjects treated of, which I have dis- 
 covered, or which have been pointed out to me 
 since the delivery of the lectures of which this 
 essay is a record. 
 
 The more extended our research becomes, the 
 more we find that knowledge is a thing of slow 
 progression, that the very notions which appear to 
 ourselves new, have arisen, though perhaps in a
 
 G CORRELATION OF PHYSICAL FORCES. 
 
 very indirect manner, from successive modifications 
 of traditional opinions. Each word we utter, each 
 thought we think, has in it the vestiges, is in itself 
 the impress, of antecedent words and thoughts. 
 As each material form, could we rightly read it, is 
 a book, containing in itself the past history of the 
 world; so, different though our philosophy may 
 now appear to be from that of our progenitors, it is 
 but theirs added to or subtracted from, transmitted 
 drop by drop through the filter of antecedent, as 
 ours will be through that of subsequent, ages. The 
 relic is to the past as is the germ to the future. 
 
 Though many valuable facts, and correct deduc- 
 tions from them, are to be found scattered amongst 
 the voluminous works of the ancient philosophers; 
 yet, giving them the credit which they pre-emi- 
 nently deserve for having devoted their lives to 
 purely intellectual pursuits, and for having thought, 
 seldom frivolously, often profoundly, nothing can 
 be more difficult than to seize and apprehend the 
 ideas of those who reasoned from abstraction to 
 abstraction who, although, as we now believe, 
 they must have depended upon observation for 
 their first inductions, afterwards raised upon them 
 such a complex superstructure of syllogistic deduc- 
 tions, that, without following the same paths, and 
 tracing the same sinuosities which led them to 
 their conclusions, such conclusions are to us unin- 
 telligible. To think as another thought, we must
 
 
 INTEODUCTORY REMARKS. 7 
 
 be placed in the same situation as he was placed : 
 the errors of commentators generally arise from 
 their reasoning upon the arguments of their text, 
 either in blind obedience to its dicta, without con- 
 sidering the circumstances under which they were 
 uttered, or in viewing the images presented to the 
 original writer from a different point to that from 
 which he viewed them. Experimental philosophy 
 keeps in check the errors both of a priori reason- 
 ing and of commentators, and. at all events, prevents 
 their becoming cumulative ; though the theories or 
 explanations of a fact be different, the fact remains 
 the same. It is, moreover, itself the exponent of 
 its discoverer's thought : the observation of known 
 phenomena have led him to elicit from nature the 
 new phenomenori : and though he may be wrong 
 in his deductions from this after its discovery, the 
 reasonings which conducted him to it are them- 
 selves valuable, and, having led from known to 
 unknown truths, can seldom be uninstructive. 
 
 Very different views existed amongst the ancients 
 as to the aims to be pursued by physical investi- 
 gation, and as to the objects likely to be attained 
 by it. I do not here mean the moral objects, 
 such as the attainment of the summum bonum, &c. 
 but the acquisitions in knowledge which such 
 investigations were likely to confer. Utility was 
 one object in view, and this was to some extent 
 attained by the progress made in astronomy and
 
 8 CORRELATION OF PHYSICAL FORCES. 
 
 mechanics ; Archimedes, for instance, seems to have 
 constantly had this end in view ; but, while pur- 
 suing natural knowledge for the sake of knowledge, 
 and the power which it brings with it, the greater 
 number seemed to entertain an expectation of 
 arriving at some ultimate goal, some point of 
 knowledge, which would give them a mastery over 
 the mysteries of nature, and would enable them to 
 ascertain what was the most intimate structure of 
 matter, and the causes of the changes it exhibits. 
 Where they could not discover, they speculated. 
 Leucippus, Democritus, and others, have given us 
 their notions of the ultimate atoms of which matter 
 was formed, and of the modus agendi of nature in 
 the various transformations which matter under- 
 goes. 
 
 The expectation of arriving at ultimate causes or 
 essences continued long after the speculations of 
 the ancients had been abandoned, and continues 
 even to the present day to be a very general no- 
 tion of the objects to be ultimately attained by 
 physical science. Francis Bacon, the great re- 
 modeller of science, entertained this notion, and 
 thought that, by experimentally testing natural 
 phenomena, we should be enabled to trace them to 
 certain primary essences or causes whence the 
 various phenomena flow. These he speaks of under 
 the scholastic name of 'forms' a term derived 
 from the ancient philosophy, but differently ap-
 
 INTRODUCTORY REMARKS. 9 
 
 plied. He appears to have understood by ' form ' 
 the essence of quality that in which, abstracting 
 everything extraneous, a given quality consists, or 
 that which, superinduced on any body, would give 
 it its peculiar quality : thus the form of trans- 
 parency is that which constitutes transparency, or 
 that by which, when discovered, transparency could 
 be produced or superinduced. To take a specific 
 example of what I may term the synthetic applica- 
 tion of his philosophy : 'In gold there meet to- 
 gether yellowness, gravity, malleability, fixedness 
 in the fire, a determinate way of solution, which 
 are the simple natures in gold ; for he who under- 
 stands form, and the manner of superinducing this 
 yellowness, gravity, ductility, fixedness, faculty of 
 fusion, solution, -&c., with their particular degrees 
 and proportions, will consider how to join them 
 together in some body, so that a transmutation 
 into gold shall follow. 
 
 On the other hand, the analytic method, or, ' the 
 enquiry from what origin gold or any other metal 
 or stone is generated from its first fluid matter or 
 rudiments, up to a perfect mineral,' is to be per- 
 ceived by what Bacon calls the latent process, or a 
 search for ' what in every generation or trans- 
 formation of bodies, flies off, what remains be- 
 hind, what is added, what separated, &c. ; also, in 
 other alterations and motions, what gives motion, 
 what governs it, and the like.' Bacon appears to
 
 10 CORRELATION OF PHYSICAL FORCES. 
 
 have thought that qualities separate from the sub- 
 stances themselves were attainable, and if not 
 capable of physical isolation, were at all events 
 capable of physical transference and superinduc- 
 tion. 
 
 Subsequently to Bacon a belief has generally 
 existed, and now to a great extent exists, in what 
 are called secondary causes, or consequential steps, 
 wherein one phenomenon is supposed necessarily to 
 hang on another, and that on another, until at last 
 we arrive at an essential cause, subject immediately 
 to the First Cause. This notion is generally pre- 
 valent both on the Continent and in this country : 
 nothing is more familiar than the expression 
 ' study the effects in order to arrive at the causes.' 
 
 Instead of regarding the proper object of phy- 
 sical science as a search after essential causes, I 
 believe it ought to be, and must be, a search after 
 facts and relations that although the word Cause 
 may be used in a secondary and concrete sense, as 
 meaning antecedent forces, yet in an abstract sense 
 it is totally inapplicable : we cannot predicate of 
 any physical agency that it is abstractedly the 
 cause of another ; arid if, for the sake of conve- 
 nience, the language of secondary causation be 
 permissible, it should be only with reference to the 
 special phenomena referred to, as it can never be 
 generalised. 
 
 The misuse, or rather varied use, of the term
 
 INTRODUCTORY REMARKS. 11 
 
 Cause, has been a source of great confusion in 
 physical theories, and philosophers are even now 
 by no means agreed as to their conception of 
 causation. The most generally received view of 
 causation, that of Hume, refers it to invariable an- 
 tecedence i. e., we call that a cause which invari- 
 ably precedes, that an effect which invariably suc- 
 ceeds. Many instances of invariable sequence might 
 however be selected, which do not present the rela- 
 tion of cause and effect : thus, as Reid observes and 
 Brown does not satisfactorily answer, day inva- 
 riably precedes night, and yet day is not the cause 
 of night. The seed, again, precedes the plant, but 
 is not the cause of it ; so that when we study phy- 
 sical phenomena it becomes difficult to separate 
 the idea of causation from that of force, and these 
 have been regarded as identical by some philoso- 
 phers. To take an example which will contrast 
 these two views : if a floodgate be raised, the water 
 flows out ; in ordinary parlance, the water is said 
 to flow because the floodgate is raised : the sequence 
 is invariable ; no floodgate, properly so called, can 
 be raised without the water flowing out, and yet 
 in another, and perhaps more strict, sense, it is the 
 gravitation of the water which causes it to flow. 
 But though we may truly say that, in this instance, 
 gravitation causes the water to flow, we cannot in 
 truth abstract the proposition and say, generally, 
 that gravitation is the cause of water flowing, as
 
 12 CORRELATION OF PHYSICAL FORCES. 
 
 water may flow from other causes, gaseous elasti- 
 city, for instance, which will cause water to flow 
 from a receiver full of air into one that is exhausted ; 
 gravitation may also, under certain circumstances, 
 arrest instead of cause the flow of water. 
 
 Upon neither view, however, can we get at any- 
 thing like abstract causation. If we regard causa- 
 tion as invariable sequence, we can find no case in 
 which a given antecedent is the only antecedent to 
 a given sequent : thus, if water could flow from no 
 other cause than the withdrawal of a floodgate, we 
 might say abstractedly that this was the cause of 
 water flowing. If, again, adopting the view which 
 looks to causation as a force, we could say that 
 water could be caused to flow only by gravitation, 
 we might say abstractedly that gravitation was the 
 cause of water flowing but this we cannot say ; 
 and if we seek and examine any other example, 
 we shall find that causation is only predicable of it 
 in the particular case, and cannot be supported as 
 an abstract proposition ; yet this is constantly at- 
 tempted. Nevertheless, in each particular case 
 where we speak of Cause, we habitually refer to 
 some antecedent power or force: we never see 
 motion or any change in matter take effect without 
 regarding it as produced by some previous change ; 
 and, when we cannot trace it to its antecedent, we 
 mentally refer it to one ; but whether this habit be 
 philosophically correct is by no means clear. In
 
 
 
 INTRODUCTORY REMARKS. 13 
 
 other words, it seems questionable, not only whether 
 cause and effect are convertible terms with ante- 
 cedence and sequence, but whether in fact cause 
 does precede effect, whether force does precede the 
 change in matter of which it is said to be the cause. 
 
 The actual priority of cause to effect has been 
 doubted, and their simultaneity argued with much 
 ability. As an instance of this argument it may 
 be said, the attraction which causes iron to approach 
 the magnet is simultaneous with and ever accom- 
 panies the movement of the iron ; the movement 
 is evidence of the coexisting cause or force, but 
 there is no evidence of any interval in time between 
 the one and the other. On this view time would 
 cease to be a necessary element in causation ; 
 the idea of cause, except perhaps as referred to a 
 primeval creation, would cease to exist; and the 
 same arguments which apply to the simultaneity of 
 cause with effect would apply to the simultaneity of 
 Force with Motion. We could not, however, even 
 if we adopted this view, dispense with the element 
 of time in the sequence of phenomena ; the effect 
 being thus regarded as ever accompanied simul- 
 taneously by its appropriate cause, we should still 
 refer it to some antecedent effect ; and our reason- 
 ing as applied to the successive production of all 
 natural changes would be the same. 
 
 Habit and the identification of thoughts with 
 phenomena so compel the use of recognised terms,
 
 14 CORRELATION OF PHYSICAL FORCES. 
 
 that we cannot avoid using the word cause even 
 in the sense in which objection is taken; and if 
 we struck it out of our vocabulary, our language, 
 in speaking of successive changes, would be unin- 
 telligible to the present generation. The common 
 error, if I am right in supposing it to be such, con- 
 sists in the abstraction of cause, and in supposing 
 in each case a general secondary cause a some- 
 thing which is not the first cause, but which, if we 
 examine it carefully, must have all the attributes 
 of a first cause, and an existence independent of, 
 and dominant over, matter. 
 
 The relations of electricity and magnetism af- 
 ford us a very instructive example of the belief in 
 secondary causation. Subsequent to the discovery 
 by Oersted of electro-magnetism, and prior to that 
 by Faraday of magneto- electricity, electricity and 
 magnetism were believed by the highest authorities 
 to stand in the relation of cause and effect i.e. 
 electricity was regarded as the cause, and mag- 
 netism as the effect; and where magnets existed 
 without any apparent electrical currents to cause 
 their magnetism, hypothetical currents were sup- 
 posed, for the purpose of carrying out the causative 
 view; but magnetism may now be said with equal 
 truth to be the cause of electricity, and electrical 
 currents may be referred to hypothetical magnetic 
 lines : if therefore electricity cause magnetism, and 
 magnetism cause electricity, why then electricity
 
 INTRODUCTORY REMARKS. 15 
 
 causes electricity, which becomes, so as to speak, a 
 reductio ad absurdum of the doctrine. 
 
 To take another instance, which may render 
 these positions more intelligible. By heating bars 
 of bismuth and antimony in contact, a current of 
 electricity is produced ; and if their extremities be 
 united by a fine wire, the wire is heated. Now 
 here the electricity in the metals is said to be 
 caused by heat, and the heat in the wire to be 
 caused by electricity, and in a concrete sense this 
 is true; but can we thence say abstractedly that 
 heat is the cause of electricity, or that electricity 
 is the cause of heat? Certainly not; for if either 
 be true, both must be so, and the effect then be- 
 comes the cause of the cause, or, in other words, 
 a thing causes itself. Any other proposition on 
 this subject will be found to involve similar dif- 
 ficulties, until, at length, the mind will become 
 convinced that abstract secondary causation does 
 not exist, and that a search after essential causes 
 is vain. 
 
 The position which I seek to establish in this 
 Essay is, that the various affections of matter 
 which constitute the main objects of experimental 
 physics, viz., heat, light, electricity, magnetism, 
 chemical affinity, and motion, are all correlative, 
 or have a reciprocal dependence: that neither, 
 taken abstractedly, can be said to be the essential 
 cause of the others, but that either may produce
 
 16 CORRELATION OF PHYSICAL FORCES. 
 
 or be convertible into, any of the others: thus 
 heat may mediately or immediately produce 
 electricity, electricity may produce heat; and 
 so of the rest, each merging itself as the force it 
 produces becomes developed: and that the same 
 must hold good of other forces, it being an irresist- 
 ible inference from observed phenomena that a 
 force cannot originate otherwise than by devolu- 
 tion from some pre-existing force or forces. 
 
 The term force, although used in very different 
 senses by different authors, in its limited sense 
 may be denned as that which produces or resists 
 motion. Although strongly inclined to believe 
 that the other affections of matter, which I have 
 above named, are and will ultimately be resolved 
 into, modes of motion, many arguments for which 
 view will be given in subsequent parts of this Essay, 
 it would be going too far, at present, to assume 
 their identity with it; I therefore use the term 
 force in reference to them, as meaning that active 
 principle inseparable from matter which is supposed 
 to induce its various changes. 
 
 The word force and the idea it aims at express- 
 ing might be objected to by the purely physical 
 philosopher on similar grounds to those which 
 apply to the word cause, as it represents a subtle 
 mental conception, and not a sensuous perception 
 or phenomenon. The objection would take some- 
 thing of this form. If the string of a bent bow be
 
 INTEODUCTOEY EEMAEKS. 17 
 
 cut, the bow will straighten itself; we thence say 
 there is an elastic/ore^ in the bow which straightens 
 it; but if we applied our expressions to this ex- 
 periment alone, the use of the term force would be 
 superfluous, and would not add to our knowledge 
 on the subject. All the information which our 
 minds could get would be as sufficiently obtained 
 from the expression, when the string is cut the 
 bow becomes straight, as from the expression, the 
 bow becomes straight by its elastic force. Do we 
 know more of the phenomenon, viewed without 
 reference to other phenomena, by saying it is 
 produced by force ? Certainly not. All we know 
 or see is the effect ; we do not see force we see 
 motion or moving matter. 
 
 If now we take a piece of caoutchouc and stretch 
 it, when released it returns to its original length. 
 Here, though the subject-matter is very different, 
 we see some analogy in the effect or phenomenon 
 to that of the strung bow. If again we suspend an 
 apple by a string, cut the string, the apple falls. 
 Here, though it is less striking, there is still an 
 analogy to the strung bow and the caoutchouc. 
 
 Now when the word force is employe J as com- 
 prehending these three different phenomena we 
 find some use in the term, not by its explaining 
 or rendering more intelligible the modus agendi of 
 matter, but as conveying to the mind something 
 which is alike in the three phenomena, however 
 c
 
 18 CORRELATION OF PHYSICAL FORCES. 
 
 distinct they may be in other respects : the word 
 becomes an abstract or generalised expression, and 
 regarded in this light is of high utility. Although 
 I have given only three examples, it is obvious 
 that the term would equally apply to 300 or 3,000 
 cases. 
 
 But it will be said, the term force is used not 
 as expressing the effect, but as that which pro- 
 duces the effect. This is true, and in this its ordi- 
 nary sense I shall use it in these pages. But though 
 the term has thus a potential meaning, to depart 
 from which would render language unintelligible, we 
 must guard against supposing that we know essen- 
 tially more of the phenomena by saying they are 
 produced by something, which something is only a 
 word derived from the constancy and similarity of 
 the phenomena we seek to explain by it. The 
 relations of the phenomena to which the terms 
 force or forces are applied give us real knowledge ; 
 these relations may be called relations of forces ; 
 our knowledge of them is not thereby lessened, 
 and the convenience of expression is greatly in- 
 creased, but the separate phenomena are not more 
 intimately known ; no further insight into why the 
 apple falls is acquired by saying it is forced to fall, 
 or it falls by the force of gravitation; by the 
 latter expression we are enabled to relate it most 
 usefully to other phenomena, but we still know no 
 more of the particular phenomenon than that under 
 certain circumstances the apple does fall. 
 
 i
 
 INTRODUCTORY REMARKS. 19 
 
 In the above illustrations, force has been treated 
 as the producer of motion, in which case the evi- 
 dence of the force is the motion produced ; thus 
 we estimate the force used to project a cannon ball 
 in terms of the mass of matter, and the velocity 
 with which it is projected. The evidence of force 
 when the term is applied to resistance to motion 
 is of a somewhat different character; the matter 
 resisting is molecularly affected, and has its struc- 
 ture more or less changed ; thus a strip of caout- 
 chouc to which a weight is suspended is elongated, 
 and its molecules are displaced as compared with 
 their position when unaffected by the gravitating 
 force. So a piece of glass bent by an appended 
 weight has its whole structure changed; this 
 internal change js made evident by transmitting 
 through it a beam of polarised light ; a relation 
 thus becomes established between the molecular 
 state of bodies and the external forces or motion of 
 masses. Every particle of the caoutchouc or glass 
 must be acting and contributing to resist or arrest 
 the motion of the mass of matter appended to it. 
 
 It is difficult, in such cases, not to recognise a 
 reality in force. We need some word to express 
 this state of tension ; we know that it produces an 
 effect, though the effect be negative in character: 
 although in this effort of inanimate matter we can 
 no more trace the mode of action to its ultimate 
 elements than we can follow out the connection 
 c -2
 
 20 CORRELATION OF PHYSICAL FORCES. 
 
 of our own muscles with the volition which calls 
 them into action, we are experimentally convinced 
 that matter changes its state by the agency of 
 other matter, and this agency we call force. 
 
 In placing the weight on the glass, we have 
 moved the former to an extent equivalent to that 
 which it would again describe if the resistance were 
 removed, and this motion of the mass becomes an 
 exponent or measure of the force exerted on the 
 glass; while this is in the state of tension, the 
 force is ever existing, capable of reproducing the 
 original motion, and while in a state of abeyance 
 as to actual motion, it is really acting on the 
 glass. The motion is suspended, but the force is 
 not annihilated. 
 
 But it may be objected, if tension or static force 
 be thus motion in abeyance, there is at all times 
 a large amount of dynamical action subtracted 
 from the universe. Every stone raised and left 
 upon a hill, every spring that is bent, and has 
 required force to upraise or bend it, has for a time, 
 and possibly for ever, withdrawn this force, and 
 annihilated it. Not so; when we raise a weight 
 and leave it at the point to which it has been 
 elevated we have changed the centre of gravity 
 of the earth, and consequently the earth's position 
 with reference to the sun, planets, and stars ; the 
 effort we have made pervades and shakes the 
 universe; nor can we present to the mind any
 
 INTRODUCTORY REMARKS. 21 
 
 exercise of force, which is not thus permanent in 
 its dynamical effects. If, instead of one weight 
 being raised, we raise two weights, each placed at 
 points of the earth diametrically opposite each other, 
 it would be said, here you have compensation, a 
 balance, no change in the centre of gravity of the 
 earth ; but we have increased the mean diameter 
 of the earth, and a perturbation of our planet, and 
 of all other celestial bodies, necessarily ensues. 
 
 The force may be said to be in. abeyance with 
 reference to the effect it would have produced, if 
 not arrested, or placed in a state of tension ; but in 
 the act of imposing this state, the relations of 
 equilibrium with other bodies have been changed, 
 and these move in their turn, so that motion of the 
 same amount would seem to be ever affecting 
 matter conceived in its totality. 
 
 Press the hands violently together; the first 
 notion may be that this is power locked up, and 
 that no change ensues. Not so ; the blood courses 
 more quickly, respiration is accelerated, changes, 
 which we may not be able to trace, take place in 
 the muscles and nerves, transpiration is increased; 
 we have given off force in various ways, and must, 
 if the effort be prolonged, replenish our sources of 
 power, by fresh chemical action in the stomach. 
 
 In books which treat of statics and dynamics, it 
 is common and perhaps necessary to isolate the 
 subjects of consideration ; to suppose, for instance,
 
 22 CORRELATION OF PHYSICAL FORCES. 
 
 two bodies gravitating, and to ignore the rest of the 
 universe. But no such isolation exists in reality, 
 nor could we predict the result if it did exist. 
 Would two bodies gravitate towards each other in 
 empty space, if space can be empty ? the notion 
 that they would is founded on the theory of attrac- 
 tion, which Newton himself repudiated, further 
 than as a convenient means of regarding the 
 subject. For purposes of instruction or argument 
 it may be convenient to assume isolated matter: 
 many conclusions so arrived at may be true, but 
 many will be erroneous. 
 
 If, in producing effects of tension or of static 
 force, the effort made pervades the universe, it may 
 be said, when the bent spring is freed, when the 
 raised weight falls, a converse series of motions 
 must be effected, and this theory would lead to a 
 mere reciprocation, which would be equally unpro- 
 ductive of permanent change with the annihilation 
 of force. If raising the weight has changed the 
 centre of gravity of the earth, and thence of the 
 universe, the fall of the weight, it will be said, 
 restores the original centre of gravity, and every- 
 thing comes back to its original status. In this 
 argument we again, in thought, isolate our experi- 
 ment; we neglect surrounding circumstances. Be- 
 tween the time of the raising and falling of the 
 weight, be the interval never so small, nay, more, 
 during the rising and during the fall, the- onrth
 
 INTRODUCTORY REMARKS. 23 
 
 has been going on revolving round its axis and 
 round the sun, to say nothing of other changes, such 
 as temperature, cosmical magnetism, &c., which 
 we may call accidental, but which, if we knew all, 
 would probably be found to be as necessary and as 
 reducible to law as the motion of the earth. A 
 change having taken place, the fall of the weight 
 does not bring back the status quo, but other 
 changes supervene, and so on. Nothing repeats 
 itself, because nothing can be placed again in the 
 same condition : the past is irrevocable.
 
 24 CORRELATION OF PHYSICAL FORCES. 
 
 MOTION. 
 
 MOTION which has been taken as the main ex- 
 ponent of force in the above examples is the 
 most obvious, the most distinctly conceived of all 
 the affections of matter. Visible motion, or rela- 
 tive change of position in space, is a phenomenon 
 so obvious to simple apprehension, that to attempt 
 to define it would be to render it more obscure ; 
 but with motion, as with all physical appearances, 
 there are certain vanishing gradations or undefined 
 limits, at which the obvious mode of action fades 
 away; to detect the continuing existence of the 
 phenomena we are obliged to have recourse to other 
 than ordinary methods of investigation, and we 
 frequently apply other and different names to the 
 effects so recognised. 
 
 Thus sound is motion ; and although in the 
 earlier periods of philosophy the identity of sound 
 and motion was not traced out, and they were con- 
 sidered distinct affections of matter indeed, at the 
 close of the last century a theory was advanced 
 that sound was transmitted by the vibrations of 
 an ether we now so readily resolve sound into
 
 MOTION. 25 
 
 motion, that to those who are familiar with acoustics, 
 the phenomena of sound immediately present to 
 the mind the idea of motion, i. e. motion of ordi- 
 nary matter. 
 
 Again, with regard to light: no doubt now 
 exists that light moves or is accompanied by 
 motion. Here the phenomena of motion are not 
 made evident by the ordinary sensuous perception, 
 as for instance the motion of a visibly moving pro- 
 jectile would be, but by an inverse deduction from 
 known relations of motion to time and space : as 
 all observation teaches us that bodies in moving 
 from one point in space to another occupy time, we 
 conclude that, wherever a continuing phenomenon 
 is rendered evident in two different points of space 
 at different times, there is motion, though we can- 
 not see the progression. A similar deduction con- 
 vinces us of the motion of electricity. 
 
 As we in common parlance speak of sound mov- 
 ing, although sound is motion, it requires no great 
 stretch of imagination to conceive light and elec- 
 tricity as motions, and not as things moving. If 
 one end of a long bar of metal be struck, a sound 
 is soon perceptible at the other end. This we 
 now know to be a vibration of the bar ; sound is 
 but a word expressive of the mode of motion 
 impressed on the bar ; so one end of a column of 
 air or glass subjected to a luminous impulse gives 
 a perceptible effect of light at the other end : this
 
 26 CORRELATION OF PHYSICAL FORCES. 
 
 can equally be conceived to be a vibration or 
 transmitted motion of particles in the transparent 
 column : this question will, however, be further dis- 
 cussed hereafter; for the present we will confine 
 ourselves to motion within the limits to which the 
 term is usually restricted. 
 
 With the perceptible phenomena of motion the 
 mental conception has been invariably associated 
 to which I have before alluded, and to which the 
 term force is given the which conception, when 
 we analyse it, refers us to some antecedent motion. 
 If we except the production of motion by heat, 
 light, &c., which will be considered in the sequel, 
 when we see a body moving we look to motion 
 having been communicated to it by matter which 
 has previously moved. 
 
 Of absolute rest Nature gives us no evidence : 
 all matter, as far as we can ascertain, is ever in 
 movement, not merely in masses, as with the pla- 
 netary spheres, but also molecularly, or throughout 
 its most intimate structure : thus every alteration 
 of temperature produces a molecular change 
 throughout the whole substance heated or cooled ; 
 slow chemical or electrical actions, actions of light 
 or invisible radiant forces, are always at play, BO 
 that as a fact we cannot predicate of any portion of 
 matter that it is absolutely at rest. Supposing, how- 
 ever, that motion is not an indispensable function 
 of matter, but that matter can be at rest, matter at
 
 MOTION. 27 
 
 rest would never of itself cease to be at rest ; it 
 would not move unless impelled to such motion 
 by some other moving body, or body which has 
 moved. This proposition applies not merely to im- 
 pulsive motion, as when a ball at rest is struck by 
 a moving body, or pressed by a spring which has 
 previously been moved, but to motion caused by 
 attractions such as magnetism or gravitation. 
 Suppose a piece of iron at rest in contact with a 
 magnet at rest ; if it be desired to move the iron 
 by the attraction of the magnet, the magnet or the 
 iron must first be moved ; so before a body falls it 
 must first be raised. A body at rest would there- 
 fore continue so for ever, and a body once in 
 motion would continue so for ever, in the same di- 
 rection and with the same velocity, unless impeded 
 by some other body, or affected by some other 
 force than that which originally impelled it. These 
 propositions may seem somewhat arbitrary, and 
 it has been doubted whether they are necessary 
 truths ; they have for a long time been received as 
 axioms, and there can at all events be no harm in 
 accepting them as postulates. It is however very 
 generally believed that if the visible or palpable 
 motion of one body be arrested by its impact on 
 another body, the motion ceases, and the force 
 which produced it is annihilated. 
 
 Now the view which I venture to submit is, that 
 force cannot be annihilated, but is merely subdi-
 
 28 CORRELATION OF PHYSICAL FORCES. 
 
 vided or altered in direction or character. First, 
 as to direction. Wave your hand: the motion, 
 which has apparently ceased, is taken up by the 
 air, from the air by the walls of the room, &c., 
 and so by direct and reacting waves, continually 
 comminuted, but never destroyed. It is true that, 
 at a certain point, we lose all means of detecting 
 the motion, from its minute subdivision, which 
 defies our most delicate means of appreciation, but 
 we can indefinitely extend our power of detecting 
 it accordingly as we confine its direction, or in- 
 crease the delicacy of our examination. Thus, if 
 the hand be moved in unconfined air, the motion 
 of the air would not be sensible to a person at a 
 few feet distance ; but if a piston of the same 
 extent of surface as the hand be moved with the 
 same rapidity in a tube, the blast of air may be 
 distinctly felt at several yards distance. There is 
 no greater absolute amount of motion in the air in 
 the second than in the first case, but its direction 
 is restrained, so as to make the means of detection 
 more facile. By carrying on this restraint, as in 
 the air-gun, we get a power of detecting the 
 motion, and of moving other bodies at far greater 
 distances. The puff of air which would in the air- 
 gun project a bullet a quarter of a mile, if allowed 
 to escape without its direction being restrained, as 
 by the bursting of a bladder, would not be percept- 
 ible at a yard distance, though the same absolute
 
 MOTION. 29 
 
 amount of motion be impressed on the surround- 
 ing air. 
 
 It may, however, be asked, what becomes of force 
 when motion is arrested or impeded by the counter- 
 motion of another body ? This is generally be- 
 lieved to produce rest, or entire destruction of 
 motion, and consequent annihilation of force : so 
 indeed it may, as regards the motion of the masses, 
 but a new force, or new character of force, now 
 ensues, the exponent of which, instead of visible 
 motion, is heat. I venture to regard the heat 
 which results from friction or percussion as a con- 
 tinuation of the force which was previously asso- 
 ciated with the moving body, and which, when 
 this impinges on another body, ceasing to exist as 
 gross, palpable motion, continues to exist as heat. 
 
 Thus, let two bodies, A and B, be supposed to 
 move in opposite directions (putting for the 
 moment out of the question all resistance, such as 
 that of the air, &c.), if they pass each other with- 
 out contact each will move on for ever in its re- 
 spective direction with the original velocity, but if 
 they touch each other the velocity of the move- 
 ment of each is reduced, and each becomes heated : 
 if this contact be slight, or such as to occasion but 
 a slight diminution of their velocity, as when the 
 surfaces of the bodies are oiled, then the heat is 
 slight ; but if the contact be such as to occasion a 
 great diminution of motion, as in percussion, or as
 
 30 CORRELATION OF PHYSICAL FORCES. 
 
 when the surfaces are roughened, then the heat is 
 great, so that in all cases the resulting heat is pro- 
 portionate to the diminished velocity. Where, 
 instead of resisting and consequently impeding 
 the motion of the body A, the body B gives way, 
 or itself takes up the motion originally communi- 
 cated to A, then we have less heat hi proportion to 
 the motion of the body B, for here the operation of 
 the force continues in the form of palpable motion : 
 thus the heat resulting from friction in the axle of 
 a wheel is lessened by surrounding it by rollers ; 
 these take up the primary motion of the axle, and 
 the less, by this means, the initial motion is im- 
 peded, the less is the resulting heat. Again, if a 
 body move in a fluid, although some heat is pro- 
 duced, the heat is apparently trifling, because the 
 particles of the fluid themselves move, and con- 
 tinue the motion originally communicated to the 
 moving body : for every portion of motion com- 
 municated to them this loses an equivalent, and 
 where both lose, then an equivalent of heat results. 
 As the converse of this proposition, it should 
 follow that the more rigid the bodies impinging on 
 each other the greater should be the amount of 
 heat developed by friction, and so we find it. Flint, 
 steel, hard stones, glass, and metals, are those 
 bodies which give the greatest amount of heat from 
 friction or percussion; while water, oil, &c., give 
 little or no heat, and from the ready mobility of
 
 MOTION. 31 
 
 their particles lessen its developement when inter- 
 posed between rigid moving bodies. Thus, if we 
 oil the axles of wheels, we have more rapid motion 
 of the bodies themselves, but less heat; if we 
 increase the resistance to motion, as by roughening 
 the points of contact, so that each particle strikes 
 against and impedes the motion of others, then we 
 have diminished motion, but increased heat ; or if 
 the bodies be smooth, but instead of sliding past each 
 other be pressed closely together and then rubbed, 
 we shall in many cases evolve more heat than by 
 the roughened bodies, as we get a greater number 
 of particles in contact and a greater resistance to 
 the initial motion. I cannot present to my mind 
 any case of heat resulting from friction which is 
 not explicable by this view : friction, according to 
 it, is simply impeded motion. The greater the 
 impediment, the more force is required to overcome 
 it, and the greater is the resulting heat ; this re- 
 sulting heat being a continuation of indestructible 
 force, capable, as we shall presently see, of reproduc- 
 ing palpable motion, or motion of definite masses. 
 
 Whatever be the nature of the bodies, rough or 
 smooth, solid or liquid, provided there be the same 
 initial force, and the whole motion be ultimately 
 arrested, there should be the same amount of heat 
 developed, though where the motion is carried on 
 through a great number of points of matter we do 
 not so sensibly perceive the resulting heat from
 
 32 CORRELATION OF PHYSICAL FORCES. 
 
 its greater dissipation. The friction of fluids pro- 
 duces heat, an effect first noticed I believe by 
 Mayer. The total heat produced by the friction 
 of fluids should, therefore, it will be said, be equal 
 to that produced by the friction of solids ; for 
 although each particle produces little heat, the 
 motion being readily taken up by the neighbour- 
 ing particles, yet by the tune the whole mass has 
 attained a state of rest there has been the same 
 impeding of the initial motion as by the friction 
 of solids if produced by the same initial force. If 
 the heat be viewed in the aggregate, and allowance 
 be made for the specific thermal capacity of the sub- 
 stances employed, it probably is the same, though 
 apparently less ; the heat in the case of solids being 
 manifested at certain defined points, while in that 
 of fluids it is dissipated, both the time and space 
 during and through which the motion is propa- 
 gated differ in the two cases, so that the heat in 
 the latter case is more readily carried off by sur- 
 rounding bodies. 
 
 If the body be elastic, and by its reaction the 
 motion impressed on it by the initial force be con- 
 tinued, then the heat is proportionately less ; and 
 were a substance perfectly elastic, and no resistance 
 opposed to it by the air or other matter, then the 
 movement once impressed would be perpetual, and 
 no heat would result. A ball of caoutchouc 
 bandied about for many minutes between a racket
 
 MOTION. 33 
 
 and a wall is not perceptibly heated, while a leaden 
 bullet projected by a gun against a wall is rendered 
 so hot as to be intolerable to the touch : in the 
 former case, the motion of the mass is continued by 
 the reaction due to its elasticity ; in the latter the 
 motion of the mass is extinguished and heat ensues. 
 A pendulum started in the exhausted receiver 
 of an air-pump continues its oscillation for hours 
 or even days ; the friction at its point of suspension 
 and the resistance of the air is minimised, and the 
 heat is therefore imperceptible, but these trifling 
 resistances in the end arrest the motion of the mass, 
 the one giving it out as heat, the other conveying 
 the force to the receiver, and thence to surrounding 
 bodies. Similar reasoning may be applied to the 
 oscillation of a coiled spring and balance wheel. 
 
 To wind up a clock a certain amount of force is 
 
 expended by the arm; this force is given back by 
 
 the descent of the weight, the wheels move, the 
 
 pendulum is kept oscillating, heat is generated at 
 
 each point of friction, and the surrounding air is 
 
 ! set in motion, a part of which is made obvious to 
 
 | us by the ticking sound. But it will be said, if 
 
 ; instead of allowing the weight to act upon the. 
 
 machinery, the cord by which it is suspended be 
 
 | cut, the weight drops and the force is at an end. 
 
 '. By no means, for in this case the house is shaken 
 
 | by the concussion, and thus the force and motion 
 
 I are continued, while in the former case the weight, 
 
 D
 
 34 COREELATION OF PHYSICAL FORCES. 
 
 reaches the ground quietly, and no evidence of 
 force or motion is manifested by its impact, the 
 whole having been previously dissipated. 
 
 If the initial motion, instead of being arrested 
 by the impact of other bodies, as in friction 
 or percussion, is impeded by confinement or com- 
 pression, as where the dilatation of a gas is 
 prevented by mechanical means, heat equally re- 
 sults : thus if a piston is used to compress air in 
 a closed vessel, the compressed air and, from it, the 
 sides of the vessel \vill be heated : the air being 
 unable to take up and carry on the original motion 
 communicates molecular motion or expansion to 
 all bodies in contact with it ; and, conversely, if we 
 expand air by mechanical motion, as by withdraw- 
 ing the piston, cold is produced. So when a solid 
 has its particles compressed or brought nearer 
 together, as when a bar of iron is hammered, heat 
 is produced beyond that which is due to percus- 
 sion alone. In this latter case we cannot very 
 easily effect the converse result, or produce cold bv 
 the mechanical dilatation of a solid, though the phe- 
 nomena of solution, where the particles of a solid 
 are detached from each other, or drawn more 
 widely asunder, give us an approximation to it : 
 in the case of solution cold is produced. 
 
 We are from a very extensive range of observa- 
 tion and experiment entitled to conclude that, with 
 some curious exceptions to be presently noticed,
 
 MOTION. 35 
 
 whenever a body is compressed or brought into 
 smaller dimensions it is heated, i. e. it expands 
 neighbouring substances. Whenever it is dilated 
 or increased in volume it is cooled, or contracts 
 neighbouring substances. 
 
 Mr. Joule has made a great number of experi- 
 ments for the purpose of ascertaining what quan- 
 tity of heat is produced by a given mechanical 
 action. His mode of experimenting is as follows. 
 An apparatus formed of floats or paddles of brass 
 or iron is made to rotate in a bath of water or mer- 
 cury. The power which gives rise to this rotation 
 is a weight raised like a clock-weight to a certain 
 height ; this by acting during its fall on a spindle 
 and pulley communicates motion to the paddle- 
 wheel, the water or mercury serving as a friction 
 medium and calorimeter ; and the heat is measured 
 by a delicate mercurial thermometer. The results 
 of his experiments he considers prove that a fall of 
 772 Ibs. through a space of one foot is able to 
 raise the temperature of one pound of water 
 through one degree of Fahrenheit's thermometer. 
 Mr. Joule's experiments are of extreme delicacy 
 he tabulates to the thousandth part of a degree of 
 Fahrenheit, and a large number of his thermo- 
 metric data are comprehended within the limits of 
 a single degree. Other experimenters have given 
 very different numerical results, but the general 
 opinion seems to be that the numbers given by 
 
 D 2
 
 36 CORRELATION OF PHYSICAL FORCES. 
 
 Mr. Joule are the nearest approximation to the 
 truth yet obtained. 
 
 Hitherto I have taken no distinction as to the 
 physical character of the bodies impinging on each 
 other ; but Nature gives us a remarkable difference 
 in the character or mode of the force eliminated 
 by friction, accordingly as the bodies which im- 
 pinge are homogeneous or heterogeneous: if the 
 former, heat alone is produced ; if the latter, elec- 
 tricity. 
 
 We find, indeed, instances given by authors, of 
 electricity resulting from the friction of homo- 
 geneous bodies; but, as I stated in my original 
 Lectures, I have not found such facts confirmed 
 by my own experiments, and this conclusion has 
 been corroborated by some experiments of Pro- 
 fessor Erman, communicated to the meeting of the 
 British Association in the year 1845, in which he 
 found that no electricity resulted from the friction 
 of perfectly homogeneous substances ; as, for in- 
 stance, the ends of a broken bar. Such experi- 
 ments as these will, indeed, be seldom free from 
 slight electrical currents, on account of the prac- 
 tical difficulty of fulfilling the condition of perfect 
 homogeneity in the substances themselves, their 
 size, their temperature, &c. ; but the effects pro- 
 duced are very trifling and vaiy in direction, and 
 the resultant effect is nought. Indeed, it would 
 be difficult to conceive the contrary. How could
 
 MOTION. 37 
 
 we possibly image to the mind or describe the 
 direction of a current from the same body to the 
 same body, or give instructions for a repetition of 
 the experiment ? It would be unintelligible to say 
 that in rubbing to and fro two pieces of bismuth, 
 iron, or glass, a current of electricity circulated from 
 bismuth to bismuth, or from iron to iron, or from 
 glass to glass; for the question immediately occurs 
 from which bismuth to which does it circulate ? 
 And should this question be answered by calling 
 one piece A, and the other B, this would only apply 
 to the particular specimens employed, the dis- 
 tinctive appellation denoting a distinction in fact, 
 as otherwise A could be substituted for B, and the 
 bar to which the positive electricity flowed would 
 in turn become the bar to which the negative elec- 
 tricity flowed. We may say that it circulates from 
 rough glass to smooth, from cast iron to wrought, 
 for here there is not homogeneity. It is more- 
 over conceivable, that when the motion is con- 
 tinuous in a definite direction, electricity may 
 result from the friction of homogeneous bodies. If 
 A and B rub against each other, revolving in 
 opposite directions, concentric currents of positive 
 and negative electricity may be conceived circu- 
 lating within the metals, and be described by 
 reference to the direction of their motion; this 
 indeed would be a different phenomenon from 
 those we have been considering ; but without some
 
 38 CORRELATION OF PHYSICAL FORCES. 
 
 distinction between the two substances in quality 
 or direction, the electrical effects are indescribable, 
 if not inconceivable. 
 
 When, however, homogeneous bodies are frac- 
 tured or even rubbed together, phenomena are ob- 
 served to which the term electricity is applied ; a 
 flash or line of light appears at the point of 
 friction which by some is called electrical, by others 
 phosphorescent. 
 
 I have myself observed a remarkable case of the 
 kind in the caoutchouc fabric now commonly used 
 for waterproof clothing: if two folds of this sub- 
 stance be allowed to cohere so as partly to unite 
 and present a difficulty of separation, then, on 
 stripping the one from the other, or tearing them 
 asunder, a line of light will follow the line of 
 separation. 
 
 If this class of phenomena be electrical, it is 
 electricity determined as it is generated ; there is no 
 dual character impressed on the matter acting, the 
 flash is electrical as a spark from the percussion of 
 flint is electrical, or as the slow combustion of 
 phosphorus, or any other case of the developement 
 of heat and light. It seems to be better to class this 
 phenomenon under the categories of heat and light 
 than under that of electricity, the latter word being 
 retained for those cases where a dual or polar 
 character of force is manifested. In experiments 
 which have been made by the friction of similar
 
 MOTION. 39 
 
 substances where the one appears positively and 
 the other negatively electrical, there will be found 
 some difference in the mode of rubbing by which 
 the molecular state of the bodies is in all probability 
 changed, making one a dissimilar substance from 
 the other ; thus it is said by Bergmann, that when 
 two pieces of glass are rubbed so that all the parts 
 of one pass over one part of the other, the former 
 is positive and the latter negative. It is obvious 
 that in this case the rubbing in one is confined to a 
 line, and that must be more altered in molecular 
 structure at the line of friction than the one where 
 the friction is spread over the whole surface: so 
 if a ribbon be drawn transversely over another 
 ribbon, the substances are not, qua the rubbing 
 action, identical ; so again, in the rupture of crys- 
 tals, we are dealing with substances having a polar 
 arrangement of particles the surfaces of the 
 fragments cannot be assumed to be molecularly 
 identical. 
 
 The developement of electricity by the common 
 electrical machine arises, as far as I can understand 
 it, from the separation or rupture of contiguity 
 betAveen dissimilar bodies; a metallic surface, the 
 amalgam of the cushion, is in contact with glass; 
 these two bodies act upon each other by the force 
 of cohesion ; and when, by an external mechanical 
 force, this is ruptured, as it is at each moment of 
 the motion of the glass plate or cylinder, electricity
 
 40 CORRELATION OF PHYSICAL FORCES. 
 
 is developed in each : were they similar bodies, heat 
 only would be developed. 
 
 According to the experiments of Mr. Sullivan 
 electricity may be produced by vibration alone if 
 the substance vibrating be composed either of 
 dissimilar metals, as a wire partly of iron and 
 partly of brass, caused to emit a musical sound, or 
 of the same metal if its parts be not homogeneous, 
 as a piece of iron one portion of which is hard 
 and crystallised and the other soft and fibrous ; the 
 current resulting appears to be due to the vibration, 
 and not to heat engendered, as it ceases immediately 
 with the vibration. 
 
 We may say, then, that in our present state of 
 knowledge, where the mutually impinging bodies 
 are homogeneous, heat and not electricity is the 
 result of friction and percussion ; where the bodies 
 impinging are heterogeneous, we may safely state 
 that electricity is always produced by friction or 
 percussion, although heat in a greater or less de- 
 gree accompanies it ; but when we come to the ques- 
 tion of the ratio in which frictional electricity is 
 produced, as determined by the different characters 
 of the substances employed, we find very complex 
 results. Bodies may differ in so many particulars 
 which influence more or less the developement of 
 electricity, such as their chemical constitution, the 
 state of their surfaces, their state of aggregation, 
 their transparency or opacity, their power of con-
 
 MOTION. 41 
 
 ducting electricity, &c., that the nor mas of their 
 action are veiy difficult of attainment. As a general 
 rule, it may be said that the developement of elec- 
 tricity is greater when the substances employed 
 are broadly distinct in their physical and chemical 
 qualities, and more particularly in their conduct- 
 ing powers ; but up to the present time the laws 
 governing such developement have not been even 
 approximately determined. 
 
 I have said, in reference to the various forces or 
 affections of matter, that either of them may, 
 mediately or immediately, produce the others ; and 
 this is all I can venture to predicate of them in 
 the present state of science ; but after much con- 
 sideration I incline strongly to the opinion that 
 science is rapidly progressing towards the esta- 
 blishment of immediate or direct relations between 
 all these forces. Where at present no immediate 
 relation is established between any of them, elec 
 tricity generally forms the intervening link or 
 middle term. 
 
 Motion, then, will directly produce heat and 
 electricity, and electricity, being produced by it, 
 will produce magnetism a force which is always 
 developed by electrical currents at right angles 
 to the direction of those currents, as will be sub- 
 sequently more fully explained. Light also is rea- 
 dily produced by motion, either directly, as when 
 accompanying the heat of friction, or mediately,
 
 42 CORRELATION OF PHYSICAL FORCES. 
 
 by electricity resulting from motion ; as in the 
 electoral spark, which has most of the attributes 
 of solar light, differing from it only in those 
 respects in which light differs when emanating 
 from different sources or seen through different 
 media; for instance, in the position of the fixed 
 h'nes in the spectrum or in the ratios of the spaces 
 occupied by rays of different refrangibility. In 
 the decompositions and compositions which the 
 terminal points proceeding from the conductors of 
 an electrical machine develope when immersed in 
 different chemical media, we get the production of 
 chemical affinity by electricity, of which motion is 
 the initial source. Lastly, motion may be again re- 
 produced by the forces which have emanated from 
 motion ; thus, the divergence of the electrometer, 
 the revolution of the electrical wheel, the deflection 
 of the magnetic needle, are, when resulting from 
 frictional electricity, palpable movements repro- 
 duced by the intermediate modes of force, which 
 have themselves been originated by motion.
 
 HEAT. 43 
 
 HEAT. 
 
 IF we now take HEAT as our starting point, we shall 
 find that the other modes of force may be readily 
 produced by it. To take motion first : this is so 
 generally, I think I may say invariably, the imme- 
 diate effect of heat, that we may almost, if not en- 
 tirely, resolve heat into motion, and view it as a 
 mechanically repulsive force, a force antagonistic to 
 attraction of cohesion or aggregation, and tending 
 to move the particles of all bodies, or to separate 
 them from each other. 
 
 It may be well here to premise, that in using the 
 terms ' particles,' or ' molecules,' which will be fre- 
 quently employed in this Essay, I do not use them 
 in the sense of the atomist, or mean to assert that 
 matter consists of indivisible particles or atoms. 
 The words will be used for the necessary purpose 
 of contradistinguishing the action of the inde- 
 finitely minute physical elements of matter from 
 that of masses having a sensible magnitude, much 
 in the same way as the terms ' lines' or ' points' 
 may be used, and with advantage in an abstract 
 sense ; though there does not exist, in fact, a thing 
 which has length and breadth without thickness,
 
 44 CORRELATION OF PHYSICAL FORCES. 
 
 and though a thing without parts or dimensions is 
 nothing. 
 
 If we put aside the sensation which heat pro- 
 duces in our own bodies, and regard heat simply in 
 relation to its effect upon inorganic matter, we find 
 that, with a very few exceptions, which I shall pre- 
 sently notice, the effects of what is called heat are 
 simply an expansion of the matter acted upon, and 
 that the matter so expanded has the power by its 
 own contraction of communicating expansion to all 
 bodies in contiguity with it. Thus, if the body be 
 a solid : for instance, iron ; a liquid, say water ; or 
 a gas, say atmospheric air each of these, when 
 heated, is expanded in every direction : in the two 
 former cases, by increasing the heat to a certain 
 point, we change the physical character of the sub- 
 stance, the solid becomes a liquid, and the liquid 
 becomes a gas ; these, however, are still expansions, 
 particularly the latter, when, at a certain period, 
 the expansion becomes rapidly and indefinitely 
 greater. But what is, in fact, commonly done in 
 order to heat a substance, or to increase the heat 
 of a substance? it is merely approximated to some 
 other heated, that is, to some other expanded sub- 
 stance, which latter is cooled or contracted as the 
 former expands. Let us now divest the mind of 
 the impression that heat is in itself anything sub- 
 stantive, and suppose that these phenomena are 
 regarded for the first time, and without any pre-
 
 HEAT. 45 
 
 conceived notions on the subject ; let us introduce 
 no hypothesis, but merely express as simply as we 
 can the facts of which we have become cognisant ; 
 to what do they amount ? to this, that matter has 
 pertaining to it a molecular repulsive power, a 
 power of dilatation, which is communicable by 
 contiguity or proximity. 
 
 Heat thus viewed, is motion, and this molecular 
 motion we may readily change into the motion of 
 masses, or motion in its most ordinary and palpable 
 form : for example, in the steam engine, the piston 
 and all its concomitant masses of matter are moved 
 by the molecular dilatation of the vapour of water. 
 
 To produce continuous motion there must be an 
 alternate action of heat and cold ; a given portion 
 of air, for instance, heated beyond the temperature 
 of the circumambient air, is expanded. If now it 
 be made to act on a movable piston, it moves this 
 to a point at which the tension or elastic force of 
 the confined air equals that of the surrounding air. 
 If the confined air be kept at this point, the 
 piston would remain stationary; but if it be 
 cooled, the external air exercising then a greater 
 relative degree of pressure, the piston returns to- 
 wards its original position; just as it will be seen, 
 when we come to the magnetic force, that a magnet 
 placed in a particular position produces motion in 
 iron near it, but to make this motion continuous, 
 or to obtain an available mechanical power, the
 
 46 CORRELATION OF PHYSICAL FORCES. 
 
 magnet must be demagnetised, or else a stable 
 equilibrium is obtained. 
 
 In the case of the piston moved by heated air 
 the motion of the mass becomes the exponent of the 
 amount of heat i. e. of the expansion or separation 
 of the molecules ; and we do not, by any of our 
 ordinary methods, test heat in any other way than 
 by its purely dynamical action. The various modi- 
 fications of the thermometer and pyrometer are all 
 measurers of heat by motion : in these instruments 
 liquid or solid bodies are expanded and elongated, 
 i. e. moved in a definite direction, and, either by 
 their own visible motion, or by the motion of an 
 attached index, communicate to our senses the 
 amount of the force by which they are moved. 
 There are, indeed, some delicate experiments which 
 tend to prove that a repulsive action between 
 separate masses is produced by heat. Fresnel 
 found that mobile bodies heated in an exhausted 
 receiver repelled each other to sensible distances ; 
 and Baden Powell found that the coloured rings 
 usually called Newton's rings change their breadth 
 and position, when the glasses between which they 
 appear are heated, in a manner which showed that 
 the glasses repelled each other. M. Faye's theoiy 
 of comets is based on some such repellent force. 
 There is, however, some difficulty in presenting 
 these phenomena to the mind in the same aspect 
 as the molecular repulsive action of heat.
 
 HEAT. 47 
 
 The phenomena of what is termed latent heat 
 have been generally considered as strongly in 
 favour of that view which regards heat either as 
 actual matter, or, at all events, as a substantive 
 entity, and not a motion or affection of ordinary 
 matter. 
 
 The hypothesis of latent matter is, I venture 
 with diffidence to think, a dangerous one it is 
 something like the old principle of Phlogiston, it is 
 not tangible, visible, audible ; it is, in fact, a mere 
 subtle mental conception, and ought, I submit, 
 only to be received on the ground of absolute ne- 
 cessity, the more so as these subtleties are apt to 
 be carried on to other natural phenomena, and so 
 they add to the hypothetical scaffolding which is 
 seldom requisite, and should be sparingly used, 
 even in the early stages of discovery. As an 
 instance, I think a striking one, of the injurious 
 effects of this, I will mention the analogous doc- 
 trine of 'invisible light;' and I do this, meaning 
 no disrespect to its distinguished author, any more 
 than, in discussing the doctrine of latent heat, I 
 can be supposed, in the slightest degree, to aim at 
 detracting from the merits of the illustrious in- 
 vestigators of the facts which that doctrine seeks to 
 explain. Is not 'invisible light,' a contradiction 
 in terms ? has not light ever been regarded as that 
 agent which affects our visual organs ? Invisible 
 light, then, is darkness, and if it exist, then is
 
 48 CORRELATION OF PHYSICAL FORCES. 
 
 darkness light. I know it may be said, that one eye 
 can detect light where another cannot ; that a cat 
 may see where a man cannot; that an insect may 
 see where a cat cannot ; but then it is not invisible 
 light to those who see it : the light, or rather the 
 object seen by the cat, may be invisible to the man, 
 but it is visible to the cat, and, therefore, cannot 
 abstractedly be said to be invisible. If we go 
 further, and find an agent which affects certain 
 substances similarly to light, but does not, as far 
 as we are aware, affect the visual organs of any 
 animal, then is it not an erroneous nomenclature 
 which calls such an agent light ? There are many 
 cases in which a deviation from the once accepted 
 meaning of words has so gradually entered into 
 common usage as to be unavoidable, but I cannot 
 but think that additions to such cases should as far 
 as possible be avoided, as injurious to that precision 
 of language which is one of the safest guards to 
 knowledge, and from the absence of which physical 
 science has materially suffered. 
 
 Let us now shortly examine the question of 
 latent heat, and see whether the phenomena may not 
 be as well, if not more satisfactorily, explained 
 without the hypothesis of latent matter, an idea 
 presenting many similar difficulties to that of 
 invisible light, though more sanctioned by usage. 
 Latent heat is supposed to be the matter of heat, 
 associated, in a masked or dormant state, with
 
 HEAT. 49 
 
 ordinary matter, not capable of being detected by 
 any test so long as the matter with which it is 
 associated remains in the same physical state, but 
 communicated to or absorbed from other bodies, 
 when the matter with which it is associated changes 
 its state. To take a common example : a pound 
 or given weight of water at 172, mixed with an 
 equal weight of water at 32, will acquire a mean 
 temperature, or 102; while water at 172, mixed 
 with an equal weight of ice at 32, will be reduced 
 to 32. By the theory of latent heat this pheno- 
 menon is thus explained : In the first case, that 
 of the mixture of water with water, both the bodies 
 being in the same physical state, no latent heat is 
 rendered sensible, or sensible heat latent ; but in 
 the second, the ice changing its condition from the 
 solid to the liquid state, abstracts from the liquid as 
 much heat as it requires to maintain it in the liquid 
 state, which it renders latent, or retains associated 
 with itself, as long as it remains liquid, but of 
 which heat no evidence can be afforded by any 
 thermoscopic test. 
 
 I believe this and similar phenomena, where 
 heat is connected with a change of state, may be 
 explained and distinctly comprehended without 
 recourse to the conception of latent heat, though it 
 requires some effort of the mind to divest itself of 
 this idea, and to view the phenomena simply in 
 their dynamical relations. To assist us in so 
 E
 
 50 CORRELATION OF PHYSICAL FORCES. 
 
 viewing them, let us first parallel with purely 
 mechanical actions, certain simple effects of heat, 
 where change of state (I mean such change as from 
 the solid to the liquid, or liquid to the gaseous 
 state) is not concerned. Thus, place within a 
 receiver a bladder, and heat the air within to 
 a higher temperature than that without it, the 
 bladder expands; so, force the air mechanically 
 into it by the air-pump, the bladder expands ; cool 
 the air on the outside, or remove its pressure me- 
 chanically by an exhausting pump, the bladder 
 also expands ; conversely, increase the external 
 repellent force, either by heat or mechanical pres- 
 sure, and the bladder contracts. In the mechanical 
 effects, the force which produced the distension is 
 derived from, and at the expense of, the mechanical 
 power employed, as from muscular force, from gra- 
 vitation, from the reacting elasticity of springs, or 
 any similar force by which the air-pump may be 
 worked. In the heating effects, the force is derived 
 from the chemical action in the lamp or source of 
 heat employed. 
 
 Let us next consider the experiment so arranged 
 that the force, which produces expansion in the 
 one case, produces a correlative contraction in the 
 other: thus, if two bladders, with a connecting 
 neck between them, be half-filled with air, as the 
 one is made to contract by pressure the other will 
 dilate, and vice versa ; so a bladder partly filled with
 
 HEAT. 51 
 
 cold air, and contained within another filled with 
 hot air, expands while the space between the blad- 
 ders contracts, exhibiting a mere transfer of the 
 same amount of repulsive force, the mobility of 
 the particles, or their mutual attraction, being the 
 same in each body ; in other words, the repulsive 
 force acts in the direction of least resistance until 
 equilibrium is produced; it then becomes a sta- 
 tic or balanced, instead of a dynamic or motive 
 force. 
 
 Let us now consider the case where a solid is to 
 be changed to a liquid, or a liquid to a gas ; here 
 a much greater amount of heat or repulsive force 
 is required, on account of the cohesion of the par- 
 ticles to be separated. In order to separate the 
 particles of the solid, precisely as much force must 
 be parted with by the warmer liquid body as keeps 
 an equal quantity of it in its liquid state; it is, 
 indeed, only with a more striking line of demarca- 
 tion, the case of the hot and cold bladder a part 
 of the repellent power of the hot particles is trans- 
 ferred to the cold particles, and separates them in 
 their turn, but the antagonist force of cohesion or 
 aggregation necessary to be overcome, being in this 
 case much stronger, requires and exhausts an 
 exactly proportionate amount of repellent force 
 mechanically to overcome it; hence the different 
 effect on a body such as the common thermometer, 
 the expanding liquid of which does not undergo a 
 
 E 2
 
 52 CORRELATION OF PHYSICAL FORCES. 
 
 similar change of state. Thus, in the example 
 above given, of the mixture of cold with hot water, 
 the hot and cold water and the mercury of the ther- 
 mometer being all in a liquid state before, and re- 
 maining so after contact, the resulting temperature 
 is an exact mean ; the hot water contracts to a certain 
 extent, the cold water expands to the same extent, 
 and the thermometer either sinks or rises the same 
 number of degrees, accordingly as it had been previ- 
 ously immersed in the cold or in the hot solution, 
 its mercury gaining or losing an equivalent of re- 
 pellent force. In the second instance, viz. the mix- 
 ture of ice with hot water, the substance we use as 
 an indicator, i.e. mercury, does not undergo the same 
 physical change as do those whose relations of volume 
 we are examining. The force viewing heat simply 
 as mechanical force which is employed in loosen- 
 ing or tearing asunder the particles of the solid ice, 
 is abstracted from the liquid water, and from the 
 liquid mercury of the thermometer, and in propor- 
 tion as this force meets with a greater resistance in 
 separating the particles of a solid than of a liquid, 
 so the bodies which yield the force suffer propor- 
 tionately a greater contraction. 
 
 If we compare the action of heat on the two 
 substances, water and mercury alone, and throw 
 out of our consideration the ice, we shall be able to 
 apply the same view: thus, if a given source of 
 heat be applied to water containing a mercurial
 
 HEAT. 53 
 
 thermometer, both the water and mercury gradually 
 expand, but in different degrees ; at a certain point 
 the attractive force of the molecules of the water is 
 so far overcome that the water becomes vapour. 
 At this point, the heat or force, meeting with much 
 less resistance from the attraction of the particles 
 of steam than from those of the mercury, expends 
 itself upon the former ; the mercury does not fur- 
 ther expand, or expands in an infinitesimally small 
 degree, and the steam expands greatly. As soon 
 as this arrives at a point where circumambient pres- 
 sure causes its resistance to further expansion to 
 be equal to the resistance to expansion in the mer- 
 cury of the thermometer, the latter again rises, and 
 so both go on expanding in an inverse ratio to 
 their molecular attractive force. If the circum- 
 ambient pressure be increased, as by confining the 
 water at the commencement of the experiment 
 within a less expansible body than itself, such as a 
 metallic chamber, then the mercury of the thermo- 
 meter continues to rise ; and if the experiment 
 were continued, the water being confined and not 
 the mercury, until we have arrived at a degree of 
 repulsive force which is able to overcome the cohe- 
 sive power of the mercury, so that this expands 
 into vapour, then we get the converse effect ; the 
 force expends itself upon the mercury, which ex- 
 pands indefinitely, as the water did in the first case, 
 and the water does not expand at all.
 
 54 CORRELATION OF PHYSICAL FORCES. 
 
 Another very usual mode of regarding the sub- 
 ject may embarrass at first sight, but a little consi- 
 deration will show that it is explicable by the same 
 doctrine. Water which has ice floating in it will 
 give, when measured by the thermometer, the same 
 temperature as the ice; i.e. both the water and 
 ice contract the mercury of the thermometer to the 
 point conventionally marked as 32. It may be 
 said, how is this reconcileable with the dynamical 
 doctrine, for according to that the solid should 
 take from the mercury of the thermometer more 
 repulsive power than the liquid; consequently, 
 the ice should contract the mercury more than the 
 water ? 
 
 My answer is, that in the proposition as thus 
 stated, the quantities of the water, ice, and mer- 
 cury are not taken into consideration, and hence a 
 necessary dynamical element is neglected: if the 
 element of quantity be included, this objection will 
 not apply. Let the thermometer, for instance, con- 
 tain 13J oz. of mercury, and stand at 100; if 
 placed in contact with an unlimited quantity of ice 
 at 32, the mercury will sink to 32. If the same 
 thermometer be immersed in an unlimited quantity 
 of water at 32, the mercury sinks also to 32; not 
 absolutely perhaps, because, however great the 
 quantity of water or ice, it will be somewhat raised 
 in temperature by the warmer mercury. This ele- 
 vation of temperature above 32, will be smaller in
 
 HEAT. 55 
 
 proportion as the quantity of water or ice is larger 
 than the quantity of mercury ; and, as we know of 
 no intermediate state between ice and water, the 
 contact of a thermometer at a temperature above 
 the freezing point with any quantity of ice exactly 
 at the freezing point would, theoretically speaking, 
 liquefy the whole, provided it had sufficient time ; 
 for as every portion of that ice would in time have 
 its temperature raised by the contact of the warmer 
 body, and as any elevation of temperature above 
 the freezing point liquefies ice, every portion should 
 be liquefied. Practically speaking, however, in 
 both cases, that of the water and of the ice, when 
 the quantity is indefinitely great the thermometer 
 falls to 32. 
 
 Now place the same thermometer at 100, suc- 
 cessively in one oz. of water at 32, and in one of 
 ice at 32 ; we shall find in the former case it will 
 be lowered only to 54, and in the latter to 32 ; 
 apply to this the doctrine of repulsive force, and 
 we get a satisfactory explanation. 
 
 In the first case, the quantity both of ice and 
 water being indefinitely great in respect to the mer- 
 cury, each reduces it to its own temperature, viz. 
 32, and the ice cannot reduce the mercury below 
 32, because the latter would receive back repulsive 
 power from the newly formed water, and this would 
 become ice ; in the second case, where the quantities 
 are limited, the mercury does lose more repulsive
 
 56 CORRELATION OF PHYSICAL FORCES. 
 
 power by the ice than by the water, and the obser- 
 vations made in reference to the first illustration 
 apply. 
 
 The above doctrine is beautifully instanced in 
 the experiment of Thilorier, by which carbonic acid 
 is solidified. Carbonic acid gas, retained in a strong 
 vessel under great pressure, is allowed to escape 
 from a small orifice; the sudden expansion requires 
 so great a supply of force, that in furnishing the de- 
 mands of the expanding gas certain other portions 
 of the gas contract to such an extent as to solidify : 
 thus, we have reciprocal expansion and contraction 
 going on in one and the same substance, the time 
 being too limited for the whole to assume a uniform 
 temperature, or in other words, a uniform extent 
 of expansion. 
 
 It has been observed with reference to heat thus 
 viewed, that it would be as correct to say, that heat 
 is absorbed, or cold produced by motion, as that heat 
 is produced by it. This difficulty ceases when the 
 mind has been accustomed to regard heat and cold 
 as themselves, motion ; i.e. as correlative expansions 
 and contractions, each being evidenced by relation, 
 and being inconceivable as an abstraction. 
 
 For instance, if the piston of an air-pump be 
 drawn down by a weight, cold is produced in the 
 receiver. It may be here said that a mechanical force, 
 and the motion consequent upon it, produces cold ; 
 but heat is produced on the opposite side of tin-
 
 
 HEAT. 57 
 
 piston, if a receiver be adapted so as to retain the 
 compressed air. Assuming them to be equivalent to 
 each other, the force of the falling weight would be 
 expressed by the heat of friction of the piston against 
 its tube, and by the tension or power of reaction of 
 the compressed against the dilated air. If the heat 
 due to compression be made to perform mechanical 
 work, it would pro tanto be consumed, and could 
 not restore the' temperature to the dilated air ; but 
 if it perform no work, no heat is lost. Mr. Joule 
 has experimentally proved this proposition. 
 
 In commencing the subject of heat, I asked my 
 reader to put out of consideration the sensations 
 which heat produces in our own bodies : I did this 
 because these sensations are likely to deceive, and 
 have deceived many as to the nature of heat. 
 These sensations are themselves occasioned by 
 similar expansions to those which we have been 
 considering ; the liquids of the body are expanded, 
 i. e. rendered less viscid by heat, and from their 
 more ready flow, we obtain the sensation of agree- 
 able warmth. By a greater degree of heat, their 
 expansion becomes too great, giving rise to a sense 
 of pain, and if pushed to extremity, as with the 
 heat which produces a burn, the liquids of the body 
 are dissipated in vapour, and an injury or destruc- 
 tion of the organic structure takes place. A similar 
 though converse effect may be produced by intense 
 cold ; the application of frozen mercury to the
 
 58 CORRELATION OF PHYSICAL FORCES. 
 
 animal body produces a burn similar to that pro- 
 duced by great heat, and accompanied with a 
 similar sensation. 
 
 Doubtless other actions than those above men- 
 tioned interfere in producing the sensations of heat 
 and cold ; but I think it will be seen that these 
 will not affect the arguments as to the nature of 
 heat. The phenomenal effects will be found un- 
 altered : heat will still be found to be expansion, 
 cold to be contraction ; and the expansion and con- 
 traction are, as with the two bladders of air, cor- 
 relative i.e. we cannot expand one body, A, with- 
 out contracting some other body, B ; we cannot con- 
 tract A without expanding B, assuming that we 
 view the bodies with relation to heat alone, and 
 suppose no other force to be manifested. 
 
 I have said that there are a few exceptions as 
 to heat being always manifested by an expansion 
 of matter. One class of these exceptions is only 
 apparent : moist clay, animal or vegetable fibre, 
 and other substances of a mixed nature, which 
 contain matter of diverse character, some of j 
 which is more and some less volatile, i.e. expan- 
 sible, are contracted on the application of heat* 
 this arises from the more volatile matter being dissi- 
 pated in the form of vapour or gas; and the inn -r- 
 stices of the less volatile being thus emptied, the 
 latter contracts by its own cohesive attraction, 
 giving thus a prirna facie appearance of contr.u--
 
 HEAT. 
 
 tion by heat. The pyrometer of Wedgwood is 
 explicable on this principle. 
 
 The second class of exceptions, though much 
 more limited in extent, is less easily explained. 
 Water, fused bismuth, and probably some other 
 substances (though the fact as to them is not 
 clearly established), expand as they approach very 
 near to the freezing or solidifying point. The 
 most probable explanation of these exceptions is, 
 that at the point of maximum density the mole- 
 cules of these bodies assume a polar or crystalline 
 condition ; that by the particles being thus arranged 
 in linear directions like chevaux de frise, interstitial 
 spaces are left, containing matter of less density, 
 so that the specific density of the whole mass is 
 diminished. 
 
 Some recent experiments of Dr. Tyndall on the 
 physical properties of ice seem to favour this view. 
 When a sunbeam, concentrated by a lens, is allowed 
 to fall on a piece of apparently homogeneous ice the 
 path of the rays is instantly studded with numerous 
 luminous spots like minute air bubbles, and the 
 planes of freezing are made manifest by these and 
 by small fissures. Stars or flower-like figures of six 
 petals appear parallel to the planes of freezing, and 
 seemingly spreading out from a central bubble. 
 These flowers are formed of water. When the 
 ice is melted in warm water no air is given off 
 from the bubbles, so they seem to be vacuous; it
 
 GO CORRELATION OF PHYSICAL FORCES. 
 
 is, however, possible that extremely minute part ides 
 of air sufficient to form foci for the melting points 
 of ice might be dissolved by the water as soon as 
 they came in contact with it. Be this as it may, 
 the existence of these points throughout the ice, 
 where it gives way to the heat of the solar beam, if 
 it does not prove actual vacuous or aeriform spaces 
 to exist in ice, proves that it is not homogeneous, 
 that its structure is probably definitely crystalline, 
 and that the matter composing it is in different 
 degrees of aggregation, so that its mean specific 
 gravity might well be less than that of water. 
 
 We cannot examine piecemeal the ultimate struc- 
 ture of matter, but in addition to the fact that the 
 bodies which evince this peculiarity are bodies 
 which, when solidified, exhibit a very marked crys- 
 talline character, there are experiments which 
 show that water between the point of maximum 
 density and its point of solidification pola rises 
 light circularly ; showing, if these experiments be 
 correct, a structural alteration in water, and one 
 analogous to that possessed by certain crystalline 
 solids, and to that possessed by water itself, where 
 it is forcibly made to assume a polarised condition 
 by the influence of magnetism. 
 
 The accuracy of these results has, however, been* 
 doubted, and the experiments have not succeeded 
 when repeated by very experienced hands. Whet her 
 this be so or not, and whether the above cxpla-
 
 HEAT. 61 
 
 nation of the exception to the otherwise invariable 
 effect of expansion by heat be or be not regarded as 
 admissible, must be left to the judgement and expe- 
 rience of each individual who thinks upon the sub- 
 ject ; at all events, no theory of heat yet proposed re- 
 moves the difficulty, and therefore it equally opposes 
 every other view of the phenomena of heat, as it 
 does that which I have here considered, and which 
 regards heat as communicable expansive force. 
 
 As certain bodies expand in freezing, and indeed, 
 under some circumstances, before they arrive at 
 the temperature at which they solidify, we get the 
 apparent anomaly that the motion or mechanical 
 force generated by heat or change of temperature 
 is reversed in direction when we arrive at the point 
 of change from the solid to the liquid state. Thus 
 a piece of ice at the temperature of Zero, Fahren- 
 heit, would expand by heat, and produce a me- 
 chanical force by such expansion until it arrives at 
 32 ; but then by an increment of heat it contracts, 
 and if the first expansion had moved a piston up- 
 wards, the subsequent contraction would bring it 
 back to a certain extent, or move it downwards, an 
 apparent negation of the force of heat. 
 
 Again with water above 40, i.e. above its 
 point of maximum density, a progressive incre- 
 ment of cold or decrement of heat would produce 
 contraction to a certain point, and then expansion 
 or a mechanical force in an opposite direction.
 
 62 CORRELATION OF PHYSICAL FORCES. 
 
 Thus not only heat or the expansive force given 
 to other bodies by a body cooling would be given 
 out by water freezing, but also the force due 
 to the expansion in the body itself, and force 
 would thus seem to be got out of nothing : but if 
 water in a confined space be gradually cooled, the 
 expansion attendant on its cooling as it approaches 
 the freezing point would occasion pressure amongst 
 its particles, and thence tend to antagonise the 
 force of dilatation produced in them by cooling, or 
 to resist their tendency to freeze ; or in other words 
 the pressure would tend to liquefaction, and con- 
 versely to the usual effect of pressure, produce cold 
 instead of heat, and thus neutralise some of the 
 heat yielded by the cooling body. Hence we find 
 that it requires a lower temperature to freeze water 
 under pressure than when exempt from it, or that 
 the freezing point is lowered as the pressure in- 
 creases for bodies which expand in freezing an 
 effect first predicted by Mr. J. Thompson, and ex- 
 perimentally verified by Mr. W. Thompson; while 
 as shown by M. Bunsen, the converse effect takes 
 place with bodies which contract in freezing. 
 Here the pressure cooperates with the effects of 
 cold, both tending to approximate the particles, 
 and such substances solidify at a liigher tempera- 
 ture in proportion as the pressure is greater; so 
 that we might expect a body <>i' this da. which 
 under the ordinary pressure of the air is at a tern-
 
 HEAT. 63 
 
 perature just above its freezing point, to solidify by 
 being submitted to pressure alone, the temperature 
 being kept constant. 
 
 A similar class of exception to the general effect 
 of heat in expanding bodies is presented by vul- 
 canised caoutchouc. This has been observed by Mr. 
 Gough, and, indeed, was pointed out to me many 
 years ago by Mr. Brockedon to be heated when 
 stretched, and cooled when unstretched. 
 
 Mr. Joule finds that its specific gravity is lower 
 when stretched than when unstretched, and that 
 when heated in its stretched state it shortens, pre- 
 senting in this particular condition a similar series 
 of relations to those which are presented by water 
 near or at its freezing point. 
 
 With the exception of this class of phenomena, 
 which offer difficulties to any theory which has 
 been proposed, the general phenomena of heat may, 
 I believe, be explained upon a purely dynamical 
 view, and more satisfactorily than by having re- 
 course to the hypothesis of latent matter. Many, 
 however, of the phenomena of heat are involved in 
 much mystery, particularly those connected with 
 specific heat or that relative proportion of heat 
 which equal weights of different bodies require to 
 raise them from a given temperature to another 
 given temperature, which appear to depend in 
 some way hitherto inexplicable upon the molecular 
 constitution of different bodies.
 
 64 CORRELATION OF PHYSICAL FORCES. 
 
 The view of heat which I have taken, viz. to 
 regard it simply as a communicable molecular 
 repulsive force, is supported by many of the pheno- 
 mena to which the term specific or relative heat is 
 applied; for example, bodies as they increase in 
 temperature increase in specific heat. The ratio 
 of this increase in specific heat is greater with 
 solids than with liquids, although the latter are 
 more dilatable ; an effect probably depending upon 
 the commencement of fusion. Again, those metals 
 whose rate of expansion increases most rapidly 
 when they are heated, increase most in specific 
 heat; and their specific heat is reduced by per- 
 cussion, which, by approximating their particles, 
 makes them specifically more dense. When, how- 
 ever, we examine substances of very different 
 physical characters, we find that their specific heats 
 have no relation to their density or rate of expan- 
 sion by heat; their differences of specific heat 
 must depend upon their intimate molecular consti- 
 tution in a manner accounted for (as far as I am 
 aware) by no theory of heat hitherto proposed. 
 
 In the greater number, probably in all solids 
 and liquids, the expansion by heat is relatively 
 greater as the temperature is higher ; or, pre- 
 serving the view of expansion and contraction, if 
 two equal portions of the same substance be juxta- 
 posed at different temperatures, the hotter portion 
 will contract a little more than the colder will
 
 HEAT. 65 
 
 expand; from this fact, viz. that the coefficient 
 of expansion increases in a given body with the 
 temperature, and from other considerations, Dr. 
 Wood has argued, with much apparent reason, that 
 the nearer the particles of bodies are to each other, 
 the less they require to move to produce a given 
 expansion or contraction in those of another body. 
 His mode of reasoning, if I rightly conceive it, may 
 be concisely put as follows : 
 
 As bodies contract by cold, it is clear that, in a 
 given body, the lower the temperature the nearer are 
 the particles; and, as the coefficient of expansion 
 increases with the temperature, the lower the tem- 
 perature of the substance be, the less the particles 
 require to move, or approach to or recede from each 
 other, so as to compensate the correlative recession 
 or approach of the particles in a hotter portion of 
 the same substance, that is in another portion of the 
 same substance in which the particles are more dis- 
 tant from each other. The amount of approxima- 
 tion or recession of the particles of a body, in other 
 words, its change of bulk by a given change of tem- 
 perature, being thus in a given substance an index 
 of the relative proximity of its particles, may it not 
 be so of all bodies ? The proposition is very in- 
 geniously argued by Dr. Wood, but the argument 
 is based upon certain hypotheses as to the. sizes and 
 distances of atoms, which must be admitted as 
 postulates by those who adopt his conclusions. Dr.
 
 66 CORRELATION OF PHYSICAL FORCES. 
 
 Wood seeks by means of this theory to explain the 
 heat produced by chemical combination, and I shall 
 endeavour to give a sketch of his mode of reasoning 
 when I arrive at that part of my subject. 
 
 Although the comparative effects of specific heat 
 may not be satisfactorily explicable by any known 
 theory, the absolute effect of heat upon each sepa- 
 rate substance is simply expansion, but when bodies 
 differing in their physical characters are used, the 
 rate of expansion varies, if measured by the corre- 
 lative contractions exhibited by the substances 
 producing it. Though I am obliged, in order to 
 be intelligible, to talk of heat as an entity, and of 
 its conduction, radiation, &c., yet these expressions 
 are, in fact, inconsistent with the dynamic theory 
 which regards heat as motion and nothing else ; 
 thus conduction would be simply a progressive di- 
 latation or motion of the particles of the conducting 
 substance, radiation an undulation or motion of the 
 particles of the medium through which the heat is 
 said to be transmitted, &c. ; and it is a strong argu- 
 ment in favour of this theory, that for every diver- 
 sity in the physical character of bodies, and for 
 every change in the structure and arrangement of 
 particles of the same body, a change is apparent in 
 the thermal effects. Thus gold conducts heat, or 
 transmits the motion called heat, more readily than 
 copper, copper than iron, iron than lead, and lead 
 than porcelain, &c.
 
 HEAT. 67 
 
 So when the structure of a substance is not 
 homogeneous, we have a change in the conduction 
 of different parts dependent upon the structure. 
 This is beautifully shown with bodies whose struc- 
 ture is symmetrically arranged, as in crystals. 
 Senannont has shown that crystals conduct heat 
 differently in different directions with reference to 
 the axis of symmetry, but definitely in definite 
 directions. His mode of experimenting is as fol- 
 lows : A plate of the crystal is cut in a direction, 
 for one set of experiments parallel, and for another 
 at right angles to the axis ; a tube of platinum is 
 inserted through the centre of the plate, and bent 
 at one extremity, so as to be capable of being 
 heated by a lamp without the heat which radiates 
 from the lamp affecting the crystal; the surfaces 
 or bases of the plate of crystal are covered with 
 wax. When the platinum is heated, the direction of 
 the heat conducted by the crystal is made known 
 by the melting of the wax, and a curved line is 
 visible at the juncture of the solid and liquid wax. 
 This curve, with homogeneous substances, as glass 
 or zinc, is a circle ; it is also a circle on plates of 
 calc spar cut perpendicular to the axis of symmetry ; 
 but on plates cut parallel to the axis of symmetry, 
 and having their plane perpendicular to one of the 
 faces of the primitive rhombohedron, the curves 
 are well-defined ellipses, having their longer axes 
 in the direction of the axis of symmetry, showing 
 
 F2
 
 6 CORRELATION OF PHYSICAL FORCES. 
 
 that this axis is a direction of greater conductibility. 
 From experiments of this character the inference is 
 drawn, that ' in media constituted like crystals of 
 the rhombohedral system, the conducting power 
 varies in such a manner, that, supposing a centre 
 of heat to exist within them, and the medium to 
 be indefinitely extended in all directions, the iso- 
 thermal surfaces are concentric ellipsoids of revo- 
 lution round the axis of symmetry, or at least 
 surfaces differing but little therefrom.' 
 
 Knoblauch has further shown, that radiant heat 
 is absorbed in different degrees, according as its 
 direction is parallel or perpendicular to the axis of 
 a crystal. 
 
 If we select a substance of a different but also 
 of a definite structure, such as wood, we find that 
 heat progresses through it with more or less rapi- 
 dity, according to its direction with reference to 
 the fibre of the wood : thus Decandolle and De la 
 Rive found that the conduction was better in a 
 direction parallel to the fibre than in one trans- 
 verse to it ; and Dr. Tyndall has added the fact, 
 that the conduction is better -in a direction trans- 
 verse to the fibre and layers of the wood than when 
 transverse to the fibre but parallel to the layers, 
 though in both these directions the conduction is 
 inferior to that following the direction of the fibre. 
 Thus, in the three possible directions in which 
 the structure of wood may be contemplated, we
 
 HEAT. 69 
 
 have three different degrees of progression for 
 heat. 
 
 In the above examples we see, as we shall see 
 farther on with reference to all the so-called im- 
 ponderables, that the phenomena depend upon the 
 molecular structure of the matter affected ; and 
 although these facts are not absolutely inconsistent 
 with the theory which supposes them to be fluids 
 or entities, it will, I think, be found to be far more 
 consistent with that which views them as motion. 
 Heat, which we are at present considering, cannot 
 be insulated: we cannot remove the heat from a 
 substance and retain it as heat ; we can only trans- 
 mit it to another substance, either as heat or as 
 some other mode of force. We only know certain 
 changes of matter, for which changes heat is a 
 generic name; the thing heat is unknown. 
 
 Heat having been shown to be a force capable of 
 producing motion, and motion to be capable of pro- 
 ducing the other modes of force, it necessarily fol- 
 lows that heat is capable, mediately, of producing 
 them: I will, therefore, content myself with en- 
 quiring how far heat is capable of immediately 
 producing the other modes of force. It will imme- 
 diately produce electricity, as shown in the beautiful 
 experiments of Seebeck, one of which I have already 
 cited, which experiments proved, that when dis- 
 similar metals are made to touch, or are soldered 
 together and heated at the point of contact, a
 
 70 CORRELATION OF PHYSICAL FORCES. 
 
 current of electricity flows through the metals 
 having a definite direction according to the metals 
 employed, which current continues as long as an 
 increasing temperature is gradually pervading the 
 metals, ceases when the temperature is stationary, 
 and flows in the contrary direction with the decre- 
 ment of temperature. 
 
 Another class of phenomena which have been 
 generally attributed to the effects of radiant heat, 
 and to which, from this belief, the term thermo- 
 graphy has been applied, may also, in their turn, 
 be made to exhibit electrical effects effects here 
 of Franklinic or static electricity, as Seebeck's 
 experiments showed effects of voltaic or dynamic 
 electricity. 
 
 If polished discs of dissimilar metals say, zinc 
 and copper be brought into close proximity, and 
 kept there for some time, and either of them has 
 irregularities upon its surface, a superficial outline 
 of these irregularities is traceable upon the other 
 disc, and vice versa. Many theories have been 
 framed to account for this phenomenon, but 
 whether it be due or not to thermic radiations, 
 the relative temperature of the discs, their relative 
 capacities and conducting and radiating powers for 
 heat, undoubtedly influence the phenomena. 
 
 Now, if two such discs in close proximity be 
 connected with a delicate electroscope, and then 
 suddenly separated, the electroscope is affected,
 
 HEAT. 71 
 
 showing that the reciprocal radiation from surface 
 to surface has produced electrical force. I cite 
 this experiment in treating of heat as an initial 
 force, because at present the probabilities are in 
 favour of thermic radiation producing the pheno- 
 menon. The origin of these so-called thermographic 
 effects is, however, a question open to much doubt, 
 and needs much further experiment. When I first 
 published the experiment which showed that the 
 mere approximation of discs of dissimilar metals 
 would give rise to electrical effects, I mentioned that 
 I considered the fact of the superficial change upon 
 the surface of metals in proximity, and, a fortiori, 
 in contact, would explain the developement of elec- 
 tricity in Volta's original contact experiment, with- 
 out having recourse to the contact theory, i. e. a 
 theory which supposes a force to be produced by 
 mere contact of dissimilar metals without any 
 molecular or chemical change. I have seen nothing 
 to alter this view. Mr. Gassiot has repeated and 
 verified my experiment with more delicate appa- 
 ratus and under more unexceptionable circum- 
 stances; and without saying that radiant heat is 
 the initial force in this case, we have evidence, by 
 the superficial change which takes place in bodies 
 closely approximated, that some molecular change 
 is taking place, some force is called into action by 
 their proximity, which produces changes in matter 
 as it expends, or rather transmits itself ; and,
 
 72 CORRELATION OF PHYSICAL FORCES. 
 
 therefore, is not a force without molecular change, 
 as the supposed contact force would be. The force 
 in this, as in all other cases, is not created, but de- 
 veloped by the action of matter on matter, and not 
 annihilated, as it is shown by this experiment to be 
 convertible into another mode of force. 
 
 To say that heat will produce light, is to assert 
 a fact apparently familiar to everyone, but there 
 may be some reason to doubt whether the expres- 
 sion to produce light is correct in this particular 
 application ; the relation between heat and light is 
 not analogous to the correlation between these and 
 the other four affections of matter. Heat and light 
 appear to be rather modifications of the same force 
 than distinct forces mutually dependent. The 
 modes of action of radiant heat and of light are so 
 similar, both being subject to the same laws of re- 
 flection, refraction, double refraction, and polarisa- 
 tion, that their difference appears to exist more in 
 the manner in which they affect our senses than in 
 our mental conception of them. 
 
 The experiments of Melloni, which have mainly 
 contributed to demonstrate the analogies of heat 
 and light, afford a beautiful instance of the assist- 
 ance which the progress of one branch of physical 
 science renders to that of another. The discoveries 
 of (Ersted and Seebeck led to the construction 
 of an instrument for measuring temperature, in- 
 comparably more delicate than any previously
 
 HEAT. 73 
 
 known. To distinguish it from the ordinary ther- 
 mometer, this instrument is called the thermo- 
 multiplier. It consists of a series of small bars of 
 bismuth and antimony, forming one zigzag chain 
 of alternations arranged parallel to each other, in 
 the shape of a cylinder or prism ; so that the 
 points of junction, which are soldered, shall be all 
 exposed at the bases of the cylinder : the two ex- 
 tremities of this series are united to a galvano- 
 meter that is, a flat coil of wire surrounding a 
 freely-suspended magnetic needle, the direction of 
 which is parallel to the convolutions of the wire. 
 When radiant heat impinges upon the soldered 
 ends of the multiplier, a thermo-electric current is 
 induced in each pair ; and, as all these currents 
 tend to circulate in the same direction, the energy 
 of the whole is increased by the cooperating forces : 
 this current, traversing the helix of the galvano- 
 meter, deflects the needle from parallelism by virtue 
 of the electro-magnetic tangential force, and the 
 degree of this deflection serves as the index of the 
 temperature. 
 
 Bodies examined by these means show a remark- 
 able difference between their transcalescence, or 
 power of transmitting heat, and their transparency : 
 thus, perfectly transparent alum arrests more heat 
 than quartz so dark coloured as to be opaque ; and 
 alum coupled with green glass Melloni found was 
 capable of transmitting a beam of brilliant light,
 
 74 CORRELATION OF PHYSICAL FORCES. 
 
 while, with the most delicate thermoscope, he could 
 detect no indications of transmitted heat: on the 
 other hand, rock-salt, the most transcalescent body 
 known, may be covered with soot until perfectly 
 opaque, and yet be found capable of transmitting a 
 considerable quantity of heat. Radiant heat, when 
 transmitted through a prism of rock-salt, is found 
 to be unequally refracted, as is the case with light; 
 and the rays of heat thus elongated into what is, 
 for the sake of analogy, called a spectrum, are found 
 to possess similar properties to the primary or co- 
 loured rays of light. Thus rock-salt is to heat what 
 colourless glass is to light ; it transmits heat of all 
 degrees of refrangibility : alum is to heat as red 
 glass to light; it transmits the least, and stops the 
 most refrangible rays ; and rock-salt covered with 
 soot represents blue glass, transmitting the most, 
 and stopping the least refrangible rays. 
 
 Certain bodies, again, reflect heat of different 
 refrangibility: thus, paper, snow, and lime, although 
 perfectly white that is, reflecting light of all de- 
 grees of refrangibility, reflect heat only of certain 
 degrees ; while metals, which are coloured bodies 
 that is, bodies which reflect light only of certain de- 
 grees of refrangibility reflect heat of all degrees. 
 Radiant heat incident upon substances which 
 doubly refract light is doubly refracted ; and the 
 emergent rays are polarised in planes at right 
 angles to each other, as is the case with light.
 
 HEAT. 75 
 
 The relation of radiation to absorption also holds 
 good with light as with heat : with the latter it has 
 been long known that the radiating power of dif- 
 ferent substances is directly proportional to their 
 absorptive and in verse to their reflective power ; or 
 rather, that the sum of the heat radiated and re- 
 flected is a constant quantity. Further, it has 
 been shown by Mr. Balfour Stewart, that the ab- 
 sorption of heat is proportional to its radiation, 
 both as to quality and quantity. 
 
 Light presents us with similar relations. Coloured 
 glass, when heated so as to be luminous, emits the 
 same light which at ordinary temperatures it 
 absorbs: thus red glass gives out or radiates a 
 greenish light, and green glass a red tint. 
 
 The flame of substances containing sodium yields 
 a yellow light of such purity that other colours 
 exposed to it appear black a phenomenon shown 
 by the familiar experiment of exposing a picture 
 of bright colours, other than yellow, to the flame of 
 spirits of wine with which common salt is mixed : 
 the picture loses its colours, and appears to be black 
 and white, When the prismatic spectrum of such a 
 flame is examined, it is found to exhibit two bright 
 yellow lines at a certain fixed position. If a source 
 of light be employed which gives no lines in its 
 spectrum, and the light of this incandescent sub- 
 stance be made to pass through the sodium flame, 
 two dark lines will appear in the spectrum precisely
 
 76 CORRELATION OF PHYSICAL FORCES. 
 
 coincident in position with the yellow lines which 
 were given by the sodium flame itself. The same 
 relation of absorption to radiation is therefore 
 shown here : the substance absorbs that light which 
 it yields when it is itself the source of light. The 
 same is true of other substances, the spectra of 
 which exhibit respectively lines of peculiar colour 
 and position. Now, the solar prismatic spectrum is 
 traversed by a great number of dark lines; and 
 Kirchhoff has deduced from considerations such as 
 those which I have shortly stated, that these dark 
 lines in the solar spectrum are due to metals 
 existing in an atmosphere around the sun, which 
 absorb the light from a central incandescent 
 nucleus, each metal absorbing that light which 
 would appear as a bright line or lines in the 
 spectrum produced by its own light. 
 
 By comparing the positions of the bright lines in 
 the spectra of various metals with those of the dark 
 lines in the solar spectrum, several of them are found 
 to be in identically the same place : hence it is in- 
 ferred, and the inference seems reasonable, that the 
 metals which show luminous lines in their spectra, 
 identical in position with dark lines in the solar spec- 
 trum, exist in the sun, and are diffused in a gaseous 
 state in its atmosphere. It does not seem to me 
 necessary to this conclusion to assume that the sun 
 is a solid mass of incandescent matter : it may well 
 be that what we term the photosphere or luminous
 
 HEAT. 77 
 
 envelope of the sun has surrounding it a more diffuse 
 atmosphere containing vaporised metals, and that 
 the mass of the sun itself may be in a different state, 
 and not necessarily at an incandescent temperature ; 
 indeed, the protuberances and red light seen at the 
 period of total eclipses afford some evidence of an 
 atmosphere exterior to the photosphere. It would, 
 however, be out of place here to speculate on 
 these subjects : the point which concerns us is the 
 analogy of heat and light, which these disco- 
 veries illustrate. Kirchhoff has carried the ana- 
 logy farther by showing that a plate of tourmaline 
 absorbs the polarised ray which when heated it 
 radiates. Thus, the phenomena of light are im- 
 itated closely by those of radiant heat; and the 
 same theory which is considered most plausibly 
 to account for the phenomena of the one, will 
 necessarily be applied to the other agent, and in 
 each case molecular change is accompanied by a 
 change in the phenomenal effects. 
 
 In certain cases heat appears to become partially 
 converted into light, by changing the matter 
 affected by heat : thus gas may be heated to a very 
 high point without producing light, or producing 
 it to a very slight degree ; but the introduction of 
 solid matter for instance, the metal platinum into 
 the highly-heated gas instantly exhibits light. 
 "Whether the heat is converted into light, or whether 
 it is concentrated and increased in intensity by
 
 78 CORRELATION OF PHYSICAL FORCES. 
 
 the solid matter so as to become visible, may be 
 open to some doubt : the fact of solid matter, 
 when ignited by the oxyhydrogen jet, decom- 
 posing water, as will be presently explained, would 
 seem to indicate that the heat was rendered more 
 intense by condensation in the solid matter, as 
 water is in this case decomposed by a heated body, 
 which body has itself been heated by the combining 
 elements of water. The apparent effect, however, 
 of the introduction of solid incombustible matter 
 into heated gas, is a conversion of heat into light. 
 
 Dr. Tyndall, by passing light from the voltaic 
 arc through solutions of iodine, separates the invisi- 
 ble rays of heat from the luminous rays, and then 
 reproduces light by receiving the former on plati- 
 num foil. 
 
 If we concentrate into a focus by a large lens a 
 dim light, we increase the intensity of the light. 
 Now if a heated body be taken, which, to the un- 
 assisted eye, has just ceased to be visible, it seems 
 probable that by collecting and condensing by a 
 lens the different rays which have so ceased to be 
 visible, light would reappear at the focus. The 
 experiment is, for reasons obvious to those ac- 
 quainted with optics, a difficult one, and, to be con- 
 clusive, should be made on a large scale, and with 
 a very perfect lens of large diameter and short 
 focus. I have obtained an approximation to the 
 result in the following manner: In a dark room
 
 HEAT. 79 
 
 a platinum wire is brought just to the point of 
 visible ignition by a constant voltaic battery; it is 
 then viewed, at a short distance, through an opera- 
 glass of large aperture applied to one eye, the other 
 being kept open. The wire will be distinctly 
 visible to that eye which regards it through the 
 opera-glass, and at the same time totally invisible 
 to the other and naked eye. It may be said with 
 some justice that such experiments prove little 
 more than the fact already known, viz. that by 
 increasing the intensity of heat, light is produced ; 
 they however exhibit this effect in a more striking 
 form, as bearing on the relations of heat and light. 
 
 With regard to chemical affinity and magnetism, 
 perhaps the only method by which in strictness the 
 force of heat may be said to produce them is 
 through the medium of electricity, the thermo- 
 electrical current, produced, as before described, 
 by heating dissimilar metals, being capable of de- 
 flecting the magnet, of magnetising iron, and ex- 
 hibiting the other magnetic effects, and also of 
 forming and decomposing chemical compounds, 
 and this in proportion to the progression of heat : 
 this has not, indeed, as yet been proved to bear a 
 measurable quantitative relation to the other forces 
 thus produced by it, because so little of the heat is 
 utilised or converted into electricity, much being 
 dissipated, without change, in the form of heat. 
 
 Heat, however, directly affects and modifies both
 
 80 CORRELATION OF PHYSICAL FORCES. 
 
 the magnet and chemical compounds ; the union of 
 certain chemical substances is induced by heat, as, 
 for instance, the formation of water by the union 
 of oxygen and hydrogen gases : in other cases this 
 union is facilitated by heat, and in many instances, 
 as in ammonia and its salts, it is weakened or an- 
 tagonised. In many of these cases, however, the 
 force of heat seems more a determining than a 
 producing influence; yet to be this, it must have 
 an immediate relation with the force whose re- 
 action it determines: thus, although gunpowder, 
 touched with an ignited wire, subsequently carries 
 on its own combustion or chemical combination, 
 independently of the original source of heat, yet 
 the chemical affinities of the first portion touched 
 must be exalted by, and at the cost of, the heat of 
 the wire; for to disturb even an unstable equili- 
 brium requires a force in direct relation with those 
 which maintain equilibrium. 
 
 Since the first edition of this essay was published, 
 I have communicated to the Royal Society some 
 experiments by which an important exception to 
 the general effect of heat on chemical affinity is 
 removed, and the results of which induce a hope 
 that a generalised relation will ultimately be 
 established between heat, chemical affinity, and 
 physical attraction. I find that if a substance 
 capable of supporting an intense heat, and in- 
 capable of being acted upon by water or either of
 
 HEAT. 81 
 
 its elements such, for instance, as platinum, or 
 iridium be raised to a high point of ignition 
 and then immersed in water, bubbles of permanent 
 gas ascend from it, which on examination are 
 found to consist of mixed oxygen and hydrogen in 
 the proportions in which they form water. The 
 temperature at which this is effected is, according 
 to Dr. Robinson, who has since written a valuable 
 paper on the subject, = 2386. Now, when mixed 
 oxygen and hydrogen are exposed to a temperature 
 of about 800, they combine and form water ; heat 
 therefore appears to act differently upon these ele- 
 ments according to its intensity, in one case pro- 
 ducing composition, in the other decomposition. 
 No satisfactory means of reconciling this apparent 
 anomaly have been pointed out : the best approxi- 
 mation to a hypothesis which I can frame is by as- 
 suming that the constituent molecules of water 
 are, below a certain temperature, in a state of 
 stable equilibrium ; that the molecules of mixed or 
 oxy hydrogen gas are, above a certain temperature, 
 also in a state of stable equilibrium, but of an op- 
 posite character ; while below this latter tempera- 
 ture the molecules of mixed gas are in a state of 
 unstable equilibrium, somewhat similar to that 
 of the fulminates or similar bodies, in which a 
 slight jderangement subverts the nicely-balanced 
 forces. 
 
 If, for instance, we suppose four molecules, 
 G
 
 82 CORRELATION OF PHYSICAL FORCES. 
 
 A, B, C, D, to be in a balanced state of equilibrium 
 between attracting and repelling forces, the ap- 
 plication of a repulsive force between B and C, 
 though it may still farther separate B and C, will 
 approximate B to A and C to D, and may bring 
 them respectively within the range of attractive 
 force ; or, supposing the repulsive force to be in 
 the centre of an indefinite sphere of particles, all 
 these, excepting those immediately acted on by 
 the force, will be approximated, and having from 
 attraction assumed a state of stable equilibrium, 
 they will retain this, because the repulsive force 
 divided by the mass is not capable of overcoming 
 it. But if the repulsive force be increased in 
 quantity and of sufficient intensity, then the at- 
 tractive force of all the molecules may be over- 
 come, and decomposition ensue. Thus, water or 
 steam below a certain temperature, and mixed gas 
 above a certain temperature, may be supposed to 
 be in the state of stable equilibrium, whilst below 
 this limiting temperature, the equilibrium of oxy- 
 hydrogen gas is unstable. 
 
 This, it must be confessed, is but a crude mode 
 of explaining the phenomena, and requires the as- 
 sumption, that the particles of a gas exercise an 
 attraction for each other as do the particles of a 
 solid, though different in degree, perhaps i kind. 
 Whether this be so or not, there can be no doubt 
 that both gases and solids expand or contract
 
 
 HEAT. 83 
 
 according to the inverse contraction or expansion 
 of other neighbouring bodies, and so far resemble 
 each other in their relations to heat and cold. The 
 extent to which such expansion or contraction can 
 be carried, seems to be limited only by the correla- 
 tive state of other bodies ; these, again, by others, 
 and so on, as far as we may judge, throughout the 
 universe. 
 
 Adopting the explanation above given of the 
 decomposition of water by heat, heat would have 
 the same relation to chemical affinity as it has to 
 physical attraction ; its immediate tendency is an- 
 tagonistic to both, and it is only by a secondary 
 action that chemical affinity is apparently pro- 
 moted by heat. This hypothesis would account 
 for heat promoting changes of the equilibrium of 
 chemical affinity among mixed compound sub- 
 stances, by decomposing certain compounds and 
 separating elementary constituents whose affinity 
 is greater, when they are brought within the 
 sphere of attraction for the substance with which 
 they are mixed, than for those with which they 
 were originally chemically united : thus an intense 
 heat being applied to a mixture of chlorine and 
 the vapour of water, occasions the production of 
 muriatic acid, liberating oxygen. 
 
 Carrying out this view, it would appear that a 
 sufficient intensity of heat might yield indefinite 
 powers of decomposition ; and there seems some 
 
 G2
 
 84 CORRELATION OF PHYSICAL FORCES. 
 
 probability of bodies now supposed to be elemen- 
 tary, being decomposed or resolved into further 
 elements by the application of heat of sufficient in- 
 tensity; or, reasoning conversely, it may fairly be 
 anticipated that bodies, which will not enter into 
 combination at a certain temperature, will enter 
 into combination if their temperature be lowered, 
 and that thus new compounds may be formed by a 
 proper disposition of their constituents when ex- 
 posed to an extremely low temperature, and the 
 more so if compression be also employed. 
 
 In considering the effect of heat as a mechanical 
 force, it would be expected, a priori, and inde- 
 pendently of any theory of heat which may be 
 adopted, that a. given amount of heat acting on a 
 given material must produce a given amount of 
 motive power; and the next question which occurs 
 to the mind is, whether the same amount of heat 
 would produce the same amount of mechanical 
 power, whatever be the material acted on or 
 affected by the heat. I will endeavour to reason 
 this out on the view of heat which I have advo- 
 cated. Heat has been considered in this essay as 
 itself motion or mechanical power, and quantity of 
 heat as measured by motion. Thus, if by a given 
 contraction of a body (say mercury) air within a 
 cylinder having a moveable piston be expanded, 
 the piston moves, and in this case the expansion or 
 motion of the material (say iron) of the cylinder
 
 HEAT. 85 
 
 itself and of the air surrounding it is commonly 
 neglected. As the air dilates it becomes colder ; 
 in other words, by undergoing expansion itself, it 
 loses its power of making neighbouring bodies ex- 
 pand; but if the piston be forcibly kept down, the 
 expansive power due to the mercury continues to 
 communicate itself to the iron and to the surround- 
 ing air, which become hotter than they would if 
 the piston had given way. 
 
 Now, in the above case, if the air be confined 
 and its volume unchanged, will the expansion of 
 the iron, assuming that it can be utilised, produce 
 an exactly equivalent mechanical effect to that 
 which the expansion of the air would produce if 
 the heat be entirely confined to it ? 
 
 Assuming that (with the exception of bodies 
 which expand in freezing, where, through a limited 
 range of temperature, the converse effects obtain) 
 whenever a body is compressed it is heated, i.e. it 
 expands neighbouring substances ; whenever it is 
 dilated or increased in volume it is cooled, i.e. it 
 contracts neighbouring substances the conclusion 
 appears to me inevitable that the mechanical power 
 produced by heat will be definite, or the same for 
 a given amount and intensity of heat, whatever be 
 the substance acted on. 
 
 Thus, let A be a definite source of heat, say a 
 pound of mercury at the temperature of 400 ; let 
 B be another equal and similar source of heat:
 
 86 CORRELATION OF PHYSICAL FORCES. 
 
 suppose A be employed to raise a piston by the 
 dilatation of air, and B to raise another piston by 
 the dilatation of the vapour of water. Imagine 
 the pistons attached to a beam, so that they oppose 
 each other's action, and thus represent a sort of 
 calorific balance. If A being applied to air could 
 conquer B, which is applied to water, it would 
 depress or throw back the piston of the latter, and, 
 by compressing the vapour, occasion an increase of 
 temperature; this, in its turn, would raise the 
 temperature of the source of heat, so that we should 
 have the anomaly that a pound of mercury at 400 
 could heat another pound of mercury at 400 to 
 401, or to some point higher than its original 
 temperature, and this without any adventitious 
 aid: it will be obvious that this is impossible, 
 at least contradictory to the whole range of our 
 experience. 
 
 The above experiment is ideal, and stated for the 
 object of giving a more precise form to the reason- 
 ing; to bring the idea more prominently into n-lirf, 
 all statements as to quantities, specific heats, &c., so 
 as to yield comparative results for given materials, 
 are omitted. The argument may be thus stated 
 in another form, viz. that by no mechanical appli- 
 ance or difference of material acted on can a given 
 source of heat be made to produce more heat than 
 it originally possessed; and that, if all be converted 
 into mechanical power, an excess cannot be
 
 HEAT. 87 
 
 supposed, for that could be converted into a surplus 
 of heat, and be a creation of force ; and a deficit 
 cannot be supposed, for that would be annihilation 
 of force. I cannot, however, see how the theoretical 
 conception could be verified by experiment; the 
 enormous weights and the complex mechanical con- 
 trivances requisite to give the measure of power 
 yielded by matter in its less dilatable forms, would 
 be far beyond our present experimental resources. 
 It would also be difficult to prevent the interference 
 of molecular attractions, inertia, &c., the overcom- 
 ing of which expends a part of the mechanical 
 power generated, but which could hardly be made 
 to appear in the result. We could not, for in- 
 stance, practically realise the above conception 
 by the construction of a machine which should 
 act by the expansion and contraction of a bar 
 of iron, and produce a power equal to that of 
 a steam engine, supplied with an equal quantity 
 of heat. 
 
 Carnot, who wrote in 1824 an essay on the 
 motive power of heat, regarded the mechanical 
 power produced by heat as -resulting from a transfer 
 of heat from one point to another, without any 
 ultimate loss of heat. Thus, in the action of an 
 ordinary steam engine, the heat from the furnace 
 having expanded the water of the boiler and raised 
 the piston, a mechanical motion is produced; but 
 this cannot be continued without the removal of
 
 88 CORRELATION OF PHYSICAL FORCES. 
 
 the heat, or the contraction of the expanded water. 
 This is done by the condenser, and the piston 
 descends. But then we have apparently trans- 
 ferred the heat from the furnace to the condenser, 
 and in the transfer effected mechanical motion. 
 
 Should the mechanical motion produced by heat 
 be considered as the effect of a simple transference 
 of heat from one point to another, or as the result 
 of a conversion of heat into the mechanical force 
 of which this motion is the result ? This question 
 leads to the following : does the heat which generates 
 the mechanical power return to the thermal ma- 
 chine as heat, or is it conveyed away by the work 
 performed ? 
 
 If a definite quantity of air be heated it is ex- 
 panded, and by its expansion it cools or loses some 
 of its power of communicating heat to neighbour- 
 ing bodies. That whicli we should have called 
 heat if the expansion of the air had been prevented, 
 we call mechanical effect, or may view as converted 
 into mechanical effect ceasing to be heat; but, 
 throwing out of the question nervous sensation, 
 this expansion or mechanical effect is all the evi- 
 dence we have of heat, for if the air is allowed to 
 expand freely, this expansion becomes the index 
 of the heat; if the air be confined, the expansion 
 of the matter of the vessel confining it, or of the 
 mercury of a thermometer hi contact with it, &c., 
 are the indices of the heat.
 
 HEAT. 89 
 
 If, again, the air which has been expanded be, 
 by mechanical pressure or by other means, restored 
 to its original bulk, it is capable of heating or ex- 
 panding other substances to a degree to which it 
 would not be equal, if it had remained in its ex- 
 panded state. To produce continuous motion, or 
 the up and down stroke of a piston, we must, as I 
 have already stated, heat and cool, just as with a 
 magnetic machine we must magnetise and demag- 
 netise in order to produce a continuous mechanical 
 effect ; and although, from the impossibility of in- 
 sulating heat, some heat is apparently lost in the 
 process, the result may be said to be effected by the 
 transfer of heat from the hot to the cold body, from 
 the furnace to the condenser. But we may equally 
 well say that the heat has been converted into me- 
 chanical force, and the mechanical force back into 
 heat ; the effects are always correlative, as are the 
 mechanical effects of an air pump, with which, as 
 we dilate the air on one side, we condense it on 
 the other ; and as we cannot dilate without the 
 reciprocal condensation, so we cannot heat without 
 the reciprocal cooling, or vice versa. 
 
 Hitherto the resistances of the piston or of any 
 superimposed weight have been thrown out of 
 consideration, or, what amounts to the same thing, 
 it has been assumed that the weight raised by 
 the piston has descended with it. The heat has 
 not merely been employed in dilating the air or
 
 90 COREELATION OF PHYSICAL FORCES. 
 
 vapour, but in raising the piston with its weight. 
 If, as the vapour is cooled, the weight be permitted 
 to descend, its mechanical force restores the heat 
 lost by the dilatation ; but in this case no part of the 
 power can be abstracted so as to be employed for 
 any practical purpose ; this question then follows, 
 what takes place with regard to the initial heat, if, 
 after the ascent of the piston, the weight be removed 
 so as not to help the piston in its descent, but to 
 fall upon a lever or produce some extraneous 
 mechanical effect? 
 
 To answer tliis question, let us suppose a weight 
 to rest on a piston which confines air at a definite 
 temperature, say for example 50, in a cylinder, the 
 whole being assumed to be absolutely non-conduct- 
 ing for heat. A part of the heat of this confined 
 air will be due to the pressure, since, as we ha\v 
 seen, compression of an elastic fluid produces heat. 
 
 Suppose, now, the confined air to be heated to 70, 
 the piston with its superincumbent weight will 
 ascend, and the temperature, in consequence of the 
 dilatation of the air, will be somewhat lowered, say 
 to 69 (we will assume, for the sake of simplicity, 
 that the heat engendered by the friction of the 
 piston compensates the force lost by friction). 
 
 The piston having reached its maximum of ele- 
 vation, let a cold body or condenser take away 20 
 from the temperature of the confined air; the 
 piston will now descend, and by the compression
 
 HEAT. 91 
 
 which the weight on it produces, will restore the 
 1 lost by dilatation, and when the piston reaches 
 its original position the temperature of the air will 
 be restored to 50. Suppose this experiment 
 repeated up to the rise of the piston ; but when the 
 piston is at its full elevation, and the cold body 
 applied, let the weight be removed, so as to drop 
 upon a wheel, or to be used for other mechanical 
 purposes. The descending piston will not now 
 reach its original point without more heat being 
 abstracted ; in consequence of the removal of the 
 weight, there will not be the same force to restore 
 the 1, and the temperature will be 49, or some 
 fraction short of the original 50. If this were 
 otherwise, then, as the weight in falling may be 
 made to produce, heat by friction, we should have 
 more heat than at first, or a creation of heat out of 
 nothing in other words, perpetual motion. 
 
 Let us now assume that this 20 supplied in the 
 first instance was yielded by a body at 90, of such 
 size and material that its total capacity for heat is 
 equal to that of the mass of confined air : this body 
 would be reduced in temperature to 70, in other 
 words, our furnace would have lost 120 of heat. 
 Let the cold body of the same size and material, 
 used as a condenser, be at 30. In the first expe- 
 riment, the body at 30 would bring back the 
 piston to its original point; but in the second 
 experiment, or that where the weight has been
 
 92 CORRELATION OF PHYSICAL FORCES. 
 
 removed, the body at 30 would not suffice to 
 restore the piston : to effect this, the cold body or 
 condenser must be at a lower temperature. 
 
 The question in Carnot's theory, which is not ex- 
 perimentally resolved, and which presents extreme 
 experimental difficulty, is the following : Granted 
 that a piston with a superimposed weight be raised 
 by the thermic expansion of confined gas or 
 vapour below it ; if the elastic medium be restored 
 to its original temperature by cooling, the weight 
 in depressing the piston will restore that portion 
 of the heat which has been lost by the expansion, 
 and by the mechanical effect consequent thereon ; 
 but if the weight be removed when at its maximum 
 of elevation, and the piston be brought back to its 
 starting point by a necessarily cooler body th:m 
 could restore it if the weight were not removed, 
 would the return of the piston now restore the 
 heat which had been lost by the dilatation, or, in 
 other words, would pulling the piston down by 
 cold restore the heat equally with the pressing it 
 down by mechanical force? The argument from 
 the impossibility of perpetual motion would say no, 
 for if all the heat were restored, the mechanical 
 effect produced by the fall of the weight, or the 
 heating effect which might be made to result from 
 this mechanical power, would be got from nothing. 
 
 Then follows another question, viz. whether, 
 where an external or derived mechanical effect
 
 HEAT. 93 
 
 has been obtained, would the return of the piston, 
 effected without the weight or external force to 
 assist it, but solely by the colder body, give to this 
 latter the same number of thermometric degrees as 
 had been lost by the hot body in the first instance ? 
 Suppose, for instance, the cold body in our experi- 
 ment to be at 20 instead of 30, would this body 
 gain 20, and then reach the temperature of 40 
 when the piston is brought back, or would its tem- 
 perature be higher or lower than 40? The argu- 
 ment from the impossibility of perpetual motion 
 does not apply here, for it does not necessarily 
 follow that 20, on the thermometric scale from 20 
 to 40, represents an equal amount of force to 20 
 on the scale from 70 to 90, and therefore it is 
 quite conceivable, that we may lose 20 from the 
 furnace, and gain 20 in the condenser, and yet have 
 obtained a certain amount of derived mechanical 
 power. It will also follow, upon a consideration of 
 the above imaginary experiments, that the greater 
 the mechanical power required, the greater should 
 be the difference between the temperature of the 
 furnace and that of the condenser; but the exact 
 relation in temperature between these for a given 
 mechanical effect, has not, as far as I am aware, 
 been satisfactorily established by experiment, 
 though it has been shown that steam at high 
 pressure produces, comparatively, a greater mecha- 
 nical effect for the same number of degrees than 
 steam at low pressure.
 
 94 CORRELATION OF PHYSICAL FORCES. 
 
 Carnot, assuming the number of degrees of tem- 
 perature to be restored, but at a lower point of the 
 theraiometric scale, termed this the fall (chute) of 
 caloric. The mechanical effect of heat, on this 
 view, may be likened to that of a series of cascades 
 on water-wheels. The highest cascade turns a 
 wheel, and produces a given mechanical effect ; the 
 water which has produced tins cannot again effect 
 it at the same level without being carried back to 
 its original elevation, i.e. without an extra force 
 being employed equivalent to, or rather a fraction 
 more than the force of the descending water ; but 
 though its power is spent with reference to the 
 first wheel, the same water may, by falling over a 
 new precipice upon a second wheel, again repro- 
 duce the same mechanical effect (strictly speaking, 
 rather more, for it has approximated the centre of 
 gravity), and so on, until no lower fall can be 
 attained. So with heat : it involves no necessity of 
 assuming perpetual motion to suppose that, after a 
 given mechanical effect, produced by a certain loss 
 of heat, the number of degrees lost from the origi- 
 nal temperature may be restored to the condenser, 
 but at a lower point of the thermometric scale. 
 
 If work has been done, i.e. if force has been 
 parted with, the original temperature itself cannot 
 be restored, but there is no a priori impossibility 
 in the same number of degrees of heat as have 
 been converted into work being conveyed to a
 
 HEAT. 95 
 
 condensing body so cold that, when it receives this 
 heat, it will still be below the original temperature 
 to which the work-producing heat was added. 
 
 In the theory of the steam-engine, this subject 
 possesses a great practical interest. Watt supposed 
 that a given weight of water required the same 
 quantity of what is termed total heat (that is, the 
 sensible added to the latent heat) to keep it in 
 the state of vapour, whatever was the pressure to 
 which it was subjected, and, consequently, however 
 its expansive force varied. Clement Desormes was 
 also supposed to have experimentally verified this 
 law. If this were so, vapour raising a piston with 
 a weight attached would produce mechanical 
 power ; and yet, the same heat existing as at first, 
 there would be no. expenditure of the initial force; 
 and if we suppose that the heat in the condenser 
 was the real representative of the original heat, we 
 should get perpetual motion. Southern supposed 
 that the latent heat was constant, and that the 
 heat of vapour under pressure increased as the 
 sensible heat. M. Despretz, in 1832, made some 
 experiments, which led him to the conclusion that 
 the increase was not in the same ratio as the sen- 
 sible heat, but that yet there was an increase ; a 
 result confirmed and verified with great accuracy 
 by M. Regnault, in some recent and elaborate 
 researches. What seems to have occasioned the 
 error in Watt and Clement Desormes' experiments
 
 96 COKRELATION OF PHYSICAL FORCES. 
 
 was, the idea involved in the term latent heat ; by 
 which, supposing the phenomenon of the disappear- 
 ance of sensible heat to be due to the absorption of 
 a material substance, that substance, ' caloric,' was 
 thought to be restored when the vapour was con- 
 densed by water, even though the water was not 
 subjected to pressure; but to estimate the total 
 heat of vapour under pressure the vapour should 
 be condensed while subjected to the same prepsuiv. 
 as that under which it is generated, as was done in 
 M. Despretz and M. Regnault's experiments. 
 
 M. Seguin, in 1839, controverted the position 
 that derived power could be got by the mere 
 transfer of heat, and by calculation from certain 
 known data, such as the law of Mariotte, viz. that 
 the elastic force of gases and vapours increased 
 directly with the pressure ; and assuming that for 
 vapour between 100 and 150 centigrade, each 
 degree of elevation of temperature was produced 
 by a thermal unit, he deduced the equivalent of 
 mechanical work capable of being performed by a 
 given decrement of heat ; and thus concluded that, 
 for ordinary pressures, about one gramme of water 
 losing one degree centigrade would produce a 
 force capable of raising a weight of 500 grammes 
 through a space of one metre; this estimate is a 
 little beyond that given by the converse experi- 
 ments of Mr. Joule, already stated, in which the 
 heat produced by a given amount of mechanical
 
 HEAT. 97 
 
 action is estimated. I am not aware that the 
 amount of mechanical work which is produced by 
 a given quantity of heat has been directly estab- 
 lished by experiment, though some approximative 
 results in particular cases have been given. Theo- 
 retically it should be the same that is to say, if 
 a fall of 772 Ibs. through a space of one foot will 
 raise the temperature of 1 Ib. of water through 
 one degree of Fahrenheit, then the fall in the tem- 
 perature of 1 Ib. of water through one degree of 
 Fahrenheit should be able to raise 772 Ibs. through 
 a space of one foot. The calculations of M. Seguiii 
 are not far from this, but since the elaborate ex- 
 periments of M. Regnault he has expressed some 
 doubt of the correctness of his former estimate, as 
 by these experiments it appears that, within certain 
 limits, for elevating the temperature of compressed 
 vapour by one degree, no more than about three- 
 tenths of a degree of total heat is required ; conse- 
 quently, the equivalent multiplied in this ratio 
 would be 1,666 grammes instead of 500. Other 
 investigators have given numbers more or less 
 discordant ; so that, without giving any opinion on 
 their different results, this question may be consi- 
 dered at present far from settled. M. Regnault 
 himself does not give the law by which the ratio of 
 heat varies with reference to the pressure, and is 
 still believed to be engaged in researches on the sub- 
 ject one involving questions of which experiments
 
 98 CORKELATION OF PHYSICAL FORCES. 
 
 on the mechanical effects of elastic fluids seem to 
 offer the most promising means of solution. 
 
 I have endeavoured to give a proof (by showing 
 the anomaly to which the contrary conclusion would 
 lead) that, whatever amount of mechanical power 
 is produced by one mode of application of heat, 
 the same should, in theory, be equally produced 
 by any other mode. But in practice the difference 
 is immense ; and therefore it becomes a question of 
 great interest practically to ascertain what is the 
 most convenient medium on which to apply the 
 heat employed, and the best machinery for econo- 
 mising it. One great problem to be solved is the 
 saving of the heat wliich the steam in ordinary 
 engines, after having done its work, carries into the 
 condenser, or, in the high-pressure engine, into the 
 air. It is argued you have a large amount of fuel 
 consumed to raise water to the boiling point, at 
 which its efficiency as a motive agent commences. 
 After it has done a small portion of work, and 
 while it still retains a very large portion of the heat 
 originally communicated to it, you reject it, and 
 have to start again with a fresh portion of steam 
 which has similarly exhausted fuel in other words, 
 you throw away all, and more than all the lu-nl 
 which has been employed in raising the water to 
 the boiling point. Various plans have been devised 
 to remedy this. Using again the warm water of 
 the condenser to feed the boiler regains a part, but
 
 HEAT. 99 
 
 a very small part, of the heat. Employing the 
 steam first for a high pressure, and then before its 
 rejection or condensation using it for a low pressure, 
 cylinder, is a second mode ; a third is to use the 
 steam, after it has done its work on the piston, as a 
 source of heat or second furnace, to boil ether, or 
 some liquid which evaporates at a lower tempera- 
 ture than water. These plans have certain advan- 
 tages ; but the complexity of apparatus, the danger 
 from combustion of 'ether, and other reasons, have 
 hitherto precluded their general adoption. Under 
 the term regenerating engine various ingenious 
 combinations have lately been suggested, and some 
 experimental engines tried, with what success it 
 is perhaps too early at present to pronounce an 
 opinion. The fundamental notion on which this 
 class of engine is based is that the vapour or 
 air, when it has performed a certain amount of 
 ork, as by raising a piston, should, instead of 
 >eing condensed or blown off, be retained and 
 gain heated to its original high temperature, and 
 len used de novo ; or that it should impart its heat 
 o some other substance, and the latter in turn 
 mpart it to the fresh vapour about to act. The 
 atter plan has been proposed by Mr. Ericsson : he 
 )asses the air which has done its work through 
 ayers of wire gauze, which are heated by the 
 ejected air, and through which the next charge of 
 r is made to pass. M. Seguin and Mr. Siemens 
 
 H 2
 
 100 CORRELATION OF PHYSICAL FORCES. 
 
 have constructed machines upon the former prin- 
 ciple, which are said to have given good experi- 
 mental results. There is, however, a theoretical 
 difficulty in all these, not affecting their capability 
 of acting, but affecting the question of economy, 
 which it does not seem easy to escape from. 
 Whether the heated air or vapour be retained, or 
 whether it yield its heat to a metallic or other 
 substance, this heat must exercise its usual repul- 
 sive force, and this must re -act cither against the re- 
 turning piston or against the incoming vapour, and 
 require a greater pressure in that to neutralise it. 
 Vapour raising a piston and producing mechanical 
 force effects this with decreasing power in proport i >n 
 as the piston is moved. At a certain point the piston 
 is arrested, or the stroke, as it is termed, is com- 
 pleted, but there is still compressed vapour in the 
 cylinder capable of doing work, but so little that it 
 is, and must in practice be neglected ; if this com- 
 pressed vapour be retained, the piston cannot lie 
 depressed without an extra force capable of over- j 
 coming the resistance of this, so to speak, semi-com- 
 pressed vapour, in addition to that which is requisite 
 to produce the normal work of the machine; and in 
 whatever way the residual force be retained, it must 
 either be antagonised at a loss of power for the 
 initial force, or at most can only yield the more 
 feeble power which it would have originally given 
 if it had been allowed to act for a longer stroke
 
 HEAT. 101 
 
 on the piston. It may be that a portion of this 
 residual force may be economised; indeed, this is 
 done when the boiler is charged with warm water 
 from the condenser, instead of with cold water ; but 
 some, indeed a notable loss, seems inevitable. 
 
 Without farther discussing the various inven- 
 tions and theories on this subject, which are daily 
 receiving increased development, it may be well 
 to point out how far nature distances art in its 
 present state. According to some careful estimates, 
 the most economical of our furnaces consume from 
 ten to twenty times as much fuel to produce the 
 same quantity of heat as an animal produces ; and 
 Matteucci found that, from a given consumption of 
 zinc in a voltaic battery, a far greater mechanical 
 effect could be produced by making it act on the 
 limbs of a recently-killed frog, notwithstanding the 
 manifold defects of such an arrangement and its 
 inferiority to the action of the living animal, than 
 when the same battery was made to produce me- 
 chanical power, by acting on an electro-magnetic 
 or other artificial motor apparatus. The ratio in 
 his experiments was nearly six to one. Thus in 
 all our artificial combinations we can but apply 
 natural forces, and with far inferior mechanism to 
 that which is perceptible in the economy of nature. 
 
 Nature is made better by no mean ; 
 But nature makes that mean ; so o'er that art, 
 Which you say adds to nature, is an art 
 That nature makes. 
 
 I JBR A Ry
 
 102 CORRELATION OF PHYSICAL FORCES. 
 
 A speculation has been thrown out by Mr. 
 Thompson, that, as a certain amount of heat re- 
 sults from mechanical action, chemical action, &c., 
 and this heat is radiated into space, there must be 
 a gradual diminution of temperature for the earth, 
 by which expenditure, however slow, being con- 
 tinuous, it would ultimately be cooled to a degree 
 incompatible with the existence of animal and 
 vegetable life in short, that the earth and t he- 
 planets of our system are parting with more heat 
 than they receive, and are therefore progressively 
 cooling. Geological researches seemed at first to 
 support this view, as they showed that the climate of 
 many portions of the terrestrial surface was at 
 remote periods hotter than at the present time : 
 the animals whose fossilised remains are found 
 in ancient strata have their organism adapted to 
 what we should now term a hot climate.* There 
 are, however, so many circumstances of difficulty 
 attending cosmical speculations, that but little reli- 
 ance can be placed upon the most profound. W- 
 know not the original source of terrestrial heat, 
 still less that of solar heat; we know not whether 
 or not systems of planets may be so constituted 
 as to communicate forces, inter se, so that forces 
 which have hitherto escaped detection may be in a 
 continuous or recurring state of interchange. 
 
 The movements produced by mutual gravitation 
 * See post, p. 315.
 
 HEAT. 103 
 
 may be the means of calling into existence mole- 
 cular forces within the substances of the planets 
 themselves. As neither from observation, nor 
 from deduction, can we fix or conjecture any 
 boundary to the universe of stellar orbs, as each 
 advance in telescopic power gives us a new shell, 
 so to speak, of stars, we may regard our globe, in 
 the limit, as surrounded by a sphere of matter 
 radiating heat, light, and possibly other forces. 
 
 Such stellar radiations would not, from the evi- 
 dence we have at present, appear sufficient to 
 supply the loss of heat by terrestrial radiations ; but 
 it is quite conceivable that the whole solar system 
 may pass through portions of space having differ- 
 ent temperatures, as was suggested, I believe, by 
 Poisson ; that as we have a terrestrial summer and 
 winter, so there may be a solar or systematic 
 summer and winter, in which case the heat lost 
 during the latter period might be restored during 
 the former. The amount of the radiations of the 
 celestial bodies may again, from changes in their 
 positions, vary through epochs which are of enor- 
 mous duration as regards the existence of the 
 human species. 
 
 The views of Mr. Thompson differ from those 
 of Laplace, recently enforced by M. Babinet, which 
 suppose the planets to have been formed by a 
 gradual condensation of nebulous matter. A modi- 
 fication of this view might, perhaps, be suggested,
 
 104 CORRELATION OF PHYSICAL FORCES. 
 
 viz. that worlds or systems, instead of being 
 created as wholes at definite periods, are gradu- 
 ally changing by atmospheric additions or subtrac- ! 
 tions, or by accretions or diminutions arising 
 from nebulous substance or from meteoric bodies, 
 so that no star or planet could at any time be. 
 said to be created or destroyed, or to be in a state 
 of absolute stability, but that some may be in- 
 creasing, others dwindling away, and so through- 
 out the universe, in the past as in the future. ' 
 When, however, questions relating to cosmogony, 
 or to the beginning or end of worlds, are con- 
 templated from a physical point of view, the peri< ><1 
 of time over which our experience, in its most en- 
 larged sense, extends, is so indefinitely minute with 
 reference to that which must be required for any 
 notable change, even in our own planet, that a 
 variety of theories may be framed equally inca- 
 pable of proof or of disproof. We have no means of 
 ascertaining whether many changes, which endure 
 in the same direction for a term beyond the range 
 of human experience, are really continuous or only 
 secular variations, which may be compensated for 
 at periods far beyond our ken, so that in such cases i 
 the question of comparative stability or change 
 can at best be only answered as to a term which, 
 though enormous with reference to our computa- 
 tions, sinks into nothing with reference to cosmiral 
 tune, if cosmical time be not eternity. Subjects
 
 HEAT 105 
 
 such as these, though of a kind on which the mind 
 delights to speculate, appear, with reference to any 
 hope of attaining reliable knowledge, far beyond 
 the reach of any present or immediately prospective 
 capacity of man.
 
 1(6 CORRELATION OF PHYSICAL FORCES. 
 
 ELECTRICITY. 
 
 ELECTRICITY is that affection of matter or mode of 
 force which most distinctly and beautifully brings 
 into relation other modes of force, and exhibits, 
 to a great extent in a quantitative form, its 
 own relation with them, and their reciprocal 
 relations with it and with each other. From 
 the manner in which the peculiar force called 
 electricity is seemingly transmitted through cer- 
 tain bodies, such as metallic wires, the term 
 current is commonly used to denote its apparent 
 progress. It is very difficult to present to the 
 mind any theory which will give a definite concep- 
 tion of its modus agendi : the early theories regard 
 its phenomena as produced either by a single fluid 
 idio-repulsive but attractive of all matter, or else 
 as produced by two fluids, each idio-repulsive but 
 attractive of the other. No substantive theory has 
 been proposed other than these two ; but although 
 this is the case, I think I shall not be unsupported 
 by many who have attentively studied electrical 
 phenomena, in viewing them as resulting, not from 
 the action of a fluid or fluids, but as a molecular 
 polarisation of ordinary matter, or as matter acting
 
 ELECTRICITY. 107 
 
 by attraction and repulsion in a definite direction. 
 Thus, the transmission of the voltaic current in 
 liquids is viewed by Grotthus as a series of chemical 
 affinities acting in a definite direction : for instance, 
 in the electrolysis of water, i.e. its decomposition 
 when placed between the poles or electrodes of a 
 voltaic battery, a molecule of oxygen is supposed 
 to be displaced by the exalted attraction of the 
 neighbouring electrode ; the hydrogen liberated by 
 this displacement unites, with the oxygen of the 
 contiguous molecule of water; this in turn liber- 
 ates its hydrogen, and so on ; the current being 
 nothing else than this molecular transmission of 
 chemical affinity. 
 
 There is strong reason for believing that, with 
 some exceptions, -such as fused metals, liquids do 
 not conduct electricity without undergoing decom- 
 position; for even in those extreme cases where a 
 trifling effect of conduction is apparently produced 
 without the usual elimination of substances at the 
 electrodes, the latter when detached fron the 
 circuit show, by the counter-current which they 
 are capable of producing when immersed in a fresh 
 liquid, that their superficial state has been changed, 
 doubtless by the determination to the surfaces of 
 minute layers of substances having opposite che- 
 mical characters. The question whether or not a 
 minute conduction in liquids can take place unac- 
 companied by chemical action, has however been
 
 108 CORRELATION OP PHYSICAL FORCEF. 
 
 much agitated, and may be regarded as inter apices 
 of the science. 
 
 Assuming for the moment electrolysis to be 
 the only known electrical phenomenon, electricity 
 would appear to consist in transmitted chemical 
 action. All the evidence we have is, that a certain 
 affection of matter or chemical change takes place 
 at certain distant points of space, the change at 
 one point having a definite relation to the change 
 at the other, and being capable of manifestation at 
 any intermediate points. 
 
 If, now, the electrical effect called induction be 
 examined, the phenomena will be found equally 
 opposed to the theory of a fluid, and consistent 
 with that of molecular polarisation. When an elec- 
 trified conductor is brought near another which 
 is not electrified, the latter becomes electrified by 
 influence or induction, as it is termed, the nearest 
 parts of each of these two bodies exhibiting states 
 of electricity of the contrary denominations. Until 
 this subject was investigated by Faraday, the in- 
 tervening non-conducting body or dielectric was 
 supposed to be purely negative, and the effect was 
 attributed to the repulsion at a distance of the 
 electrical fluid. Faraday showed that these effects 
 differed greatly according to the dielectric that 
 was interposed. Thus they were more exalted with 
 sulphur than with shellac; more with shcllm- 
 than with glass, &c. Matteucci, though differing
 
 ELECTRICITY. 109 
 
 from Faraday as to the explanation he gave, added 
 some experiments which prove that the intervening 
 dielectic is molecularly polarised. Thus a number 
 of thin plates of mica are superposed like a pack 
 of cards; metallic plates are applied to the outer 
 facings, and one of them electrified, so that the 
 apparatus is charged like a Leyden phial. Upon 
 separating the plates with insulating handles, each 
 plate is separately electrified, one side of it being 
 positive and the other negative, showing very 
 neatly and decisively a polarisation throughout the 
 intervening substances by the effect of induction. 
 
 Indeed, chemical action or electrolysis may, as 
 I have shown, be transmitted by induction across 
 a dielectric substance, such as glass, but apparently 
 only while the glass is being charged with electri- 
 city. A wire passing through and hermetically 
 sealed into a glass tube, a short portion only pro- 
 jecting, is made to dip into water contained in a 
 Florence flask; the flask is immersed in water to 
 an equal depth with that within it ; the wire and 
 another similar wire dipping into the outer water 
 are made to communicate metallically with the 
 powerful electrical machine known as Rhumkorf 's 
 coil; bubbles of gas instantly ascend from the 
 exposed portions of the wires, but cease after a 
 certain time, and are renewed when, after an inter- 
 val of separation, the coil is again connected with 
 the wires.
 
 110 CORRELATION OF PHYSICAL FORCES. 
 
 The following interesting experiment by Mr. 
 Karsten goes a step farther in corroboration of the 
 molecular changes consequent upon electrisation: 
 A coin is placed on a pack of thin plates of glass, 
 and then electrified. On removing the coin and 
 breathing on the glass plate, an impression of the 
 coin is perceptible ; this shows a certain molecular 
 change on the surface of the glass opposed to the 
 plate, or of the vapours condensed on such surface. 
 This effect might, and has been interpreted as 
 arising from a film of greasy deposit, supposed to 
 exist on the plate ; the impressions, however, have 
 been proved to penetrate to certain depths below 
 the surface, and not to be removed by polishing. 
 
 The following experiment, however, goes farther: 
 On separating carefully the glass plates, images of 
 the coin can be developed on each of the surfaces, 
 showing that the molecular change has been trans- 
 mitted through the substance of the glass ; and we 
 may thence reasonably suppose that a piece of 
 glass, or other dielectric body, if it could be split 
 up while under the influence of electric induction, 
 would exhibit some molecular change at each side 
 of each lamina, however minutely subdivided. I 
 have succeeded in farther extending this experi- 
 ment, and in permanently fixing the images thus 
 produced by electricity. Between two carefully- 
 cleaned glass plates is placed a word or device cut 
 out of paper or tinfoil ; sheets of tinfoil a little
 
 ELECTRICITY. Ill 
 
 smaller than the glass plates are placed on the 
 outside of each plate, and these coatings are 
 brought into contact with the terminals of 
 Rhumkorfs coil. After electrisation for a few 
 seconds, the glasses are separated, and their inte- 
 rior surfaces exposed to the vapour of hydrofluoric 
 acid, which acts chemically on glass ; the portions 
 of the glass not protected by the paper device are 
 corroded, AYhile those so protected are untouched 
 or less affected by the acid, so that a permanent 
 etching is thus produced, which nothing but dis- 
 integration of the glass will efface. 
 
 Some further experiments of mine on this subject 
 bring out in a still more striking manner these cu- 
 rious molecular changes. One of the plates of glass 
 having been electrified in the manner just men- 
 tioned, is coated, on the side impressed with the 
 invisible electrical image, with a film of iodised collo- 
 dion in the manner usually adopted for photographic 
 purposes ; it is then in a dark room immersed in a 
 solution of nitrate of silver ; then exposed to diffuse 
 light for a few seconds. On pouring over the collo- 
 dion the usual solution of pyrogallic acid, the invi- 
 sible electrical image is brought out as a dark device 
 on a light ground, and can be permanently fixed by 
 hyposulphite of soda. The point worthy of ob- 
 servation in this experiment is, that this perma- 
 nent image exists in the collodion film, which can 
 be stripped off the glass, dried, and placed on any
 
 112 CORRELATION OF PHYSICAL FORCES. 
 
 other surface, so that the molecular change con- 
 sequent on electrisation has communicated, by 
 contact or close proximity, a change to the film 
 of collodion corresponding in form with that on 
 the glass, but undoubtedly of a chemical nature. 
 Electricity has, moreover, in this experiment so 
 modified the surface of glass, that it can, in its 
 turn, modify the structure of another substance so 
 as to alter the relation of the latter to light. It 
 would require a curious complication of hypothetic 
 fluids to explain this; but if electricity and light 
 be supposed to be affections of ordinary ponderable 
 matter, the difficulty is only one of detail. 
 
 If, again, we examine the electricity of the atmo- 
 sphere, when, as is usually the case, it is positive 
 with respect to that of the earth, we find that each 
 successive stratum is positive to those below it and 
 negative to those above it; and the converse is t In- 
 case when the electricity of the atmosphere is m - 
 gative with respect to that of the earth. 
 
 If another electrical phenomenon be selected, 
 another sort of change will be found to have taken 
 place. The electric spark, the brush, and similar 
 phenomena, the old theories regarded as actual 
 emanations of the matter or fluid, Electricity; 
 venture to regard them as produced by an emissi 
 of the material itself from whence they issue, and 
 molecular action of the gas, or intermedium, through 
 or across which they are transmitted. 
 
 ;.: 
 ';:
 
 ELECTRICITY. 113 
 
 The colour of the electric spark, or of the voltaic 
 arc (i.e. the flame which plays between the ter- 
 minal points of a powerful voltaic battery), is de- 
 pendent upon the substance of the metal, subject 
 to certain modifications of the intermedium : thus, 
 the electric spark or arc from zinc is blue; from 
 silver, green ; from iron, red and scintillating ; pre- 
 cisely the colours afforded by these metals in their 
 ordinary combustion. A portion of the metal is also 
 found to be actually transmitted with every electric 
 or voltaic discharge: in the latter case, indeed, 
 where the quantity of matter acted upon is greater 
 than in the former, the metallic particles emitted 
 by the electrodes or terminals can be readily col- 
 lected, tested, or even weighed. It would thus 
 appear that the electrical discharge arises, at least 
 in part, from an actual repulsion and severance of 
 the electrified matter itself, which flies off" at the 
 points of least resistance. 
 
 A careful examination of the phenomena at- 
 tending the electric spark or the voltaic arc, which 
 latter is the electric disruptive discharge acting on 
 greater portions of matter, tends to modify con- 
 siderably our previous idea of the nature of the 
 electric force as a producer of ignition and combus- 
 tion. The voltaic arc is perhaps, strictly speaking, 
 neither ignition nor combustion. It is not simply 
 ignition ; because the matter of the terminals is not 
 merely brought to a state of incandescence, but is
 
 114 CORRELATION OF PHYSICAL FORCES. 
 
 physically separated and partially transferred from 
 one electrode to another, much of it being dis- 
 sipated in a vaporous state. It is not combustion; 
 for the phenomena will take place independently of 
 atmospheric air, oxygen gas, or any of the bodies 
 usually called supporters of combustion, combus- 
 tion being in fact chemical union attended with 
 heat and light. In the voltaic arc we may have no 
 chemical union ; for if the experiment be performed 
 in an exhausted receiver, or in- nitrogen, the sub- 
 stance forming the electrodes is condensed, and 
 precipitated upon the interior of the vessel in, che- 
 mically speaking, an unaltered state. Thus, to take 
 a very striking example, if the voltaic discharge 
 be taken between zinc terminals in an exhausted 
 receiver, a fine black powder of zinc is deposited on 
 the sides of the receiver ; this can be collected, and 
 takes fire readily in the air by being touched with 
 a match, or ignited wire, instantly burning into 
 white oxide of zinc. To an ordinary observer, the 
 zinc would appear to be burned twice first in the 
 receiver, where the phenomenon presents all the 
 appearance of combustion, and secondly in the 
 real combustion in air. With iron the experiment 
 is equally instructive. Iron is volatilised by the 
 voltaic arc in nitrogen or in an exhausted receiver? 
 and when a scarcely perceptible film has lined the 
 receiver, this is washed with an acid, which then 
 gives, with ferrocyanide of potassium, the prus
 
 ELECTK1CITY. 115 
 
 blue precipitate. In this case we readily distil 
 iron, a metal by ordinary means fusible only at a 
 very high temperature. 
 
 Another strong evidence that the voltaic dis- 
 charge consists of the material itself of which the 
 terminals are composed, is the peculiar rotation 
 which is observed in the light when iron is em- 
 ployed, the magnetic character of this metal 
 causing its molecules to rotate by the influence of 
 the voltaic current. 
 
 If we increase the number of reduplications in 
 a voltaic series, we increase the length of the arc, 
 and also increase its intensity or power of over- 
 coming resistance. With a battery consisting of a 
 limited number, say 100 reduplications, the dis- 
 charge will not pass from one terminal to the other 
 without first bringing them into contact, but if we 
 increase the number of cells to 400 or 500, the 
 discharge will pass from one terminal to the other 
 before they are brought into contact. The differ- 
 ence between what is called Franklinic electricity, 
 or that produced by an ordinary electrical machine, 
 and voltaic electricity, or that produced by the 
 ordinary voltaic battery, is that the former is of 
 much greater intensity than the latter, or has a 
 greater power of overcoming resistance, but, as- 
 suming an equal initial power, it acts upon a much 
 smaller quantity of matter. If, then, a voltaic 
 batteiy be formed with a view to increase the 
 
 i 2
 
 116 CORRELATION OF PHYSICAL FORCES. 
 
 intensity and lessen the quantity, the character 
 of the electrical phenomena approximate those of 
 the electrical machine. In order to effect this, the 
 sizes of the plates of the battery, and thence the 
 quantity of matter acted on in each cell, must be re- 
 duced, but the number of reduplications increased. 
 Thus if in a battery of 100 pairs of plates each pinto 
 be divided, and the battery be arranged so as to 
 form 200 pairs, each being half the original ni/o, 
 the quantitative effects are diminished, and the 
 effects of intensity increased. By carrying on this 
 sub-division, diminishing the sizes and increasing 
 the number, as is the case in the voltaic piles of 
 Deluc and Zamboni, effects are ultimately pro- 
 duced similar to those of Franklinic electricity, and 
 we thus gradually pass from the voltaic arc to the 
 spark or electric discharge. 
 
 This discharge, as I have already stated, has a 
 colour depending in part upon the nature of the 
 terminals employed. If these terminals be highly 
 polished, a spot will be observed, even in the case 
 of a small electric spark, at the points from which 
 the discharge emanates. The matter of the termi- 
 nals is itself affected ; and a transmission of this mat- 
 ter across the intervening space is detected by the 
 deposition of minute quantities of the metal or sub- 
 stance composing the one, upon the other termiiinl. 
 
 If the gas or elastic medium between the ter-1 
 minals be changed, a change takes place hi the!
 
 ELECTRICITY. 117 
 
 length or colour of the discharge, showing an 
 affection of the intervening matter. If the gas be 
 rarefied, the discharge gradually changes with the 
 degree of rarefaction, from a spark to a luminous 
 glow or diffuse light, differing in colour in dif- 
 ferent gases, and capable of extending to a much 
 greater distance than when it takes place in air of 
 the ordinary density. Thus, in highly-attenuated 
 air a discharge may be made to pass across six or 
 seven feet of space, while in air of the ordinary 
 density it would not pass across an inch. An ob- 
 server regarding the beautiful phenomena exhibited 
 by this electric discharge in attenuated gas, which, 
 from some degree of similarity in appearance to 
 the Aurora Borealis, has been called the electric 
 Aurora, would have some difficulty in believing 
 such effects could be due to an action of ordinary 
 matter. The amount of gas present is extremely 
 small; and the terminals, to a cursory examination, 
 show no change after long experimenting. It is 
 therefore not to be wondered at that the first 
 observers of this and similar phenomena, regarded 
 electricity as in itself something as a specific 
 existence or fluid. Even in this extreme case, 
 however, upon a more careful examination we shall 
 find that a change does take place, both as regards 
 the gas and as regards the terminals. Let one of 
 these consist of a highly -polished metal a silver 
 plate is one of the best materials for the purpose
 
 118 CORRELATION OF PHYSICAL FORCES. 
 
 and let the discharges in attenuated atmospheric 
 air take place from a point, say a common sewing 
 needle, to the surface of the polished silver plate; 
 it will be found, that tin's is gradually changed in 
 appearance opposite the point it is oxidated, and 
 gradually more and more corroded as the discharge 
 is continued. 
 
 If now the gas be changed, and highly-rarefied 
 hydrogen be substituted for the rarefied air, all 
 other things remaining the same, upon passing the 
 discharges as before the oxide will be cleared off 
 the plate, and the polish to a great extent restored 
 not entirely, because the silver has been disinte- 
 grated by the oxidation and the portion which has 
 been aifected by the discharge will present a some- 
 what different appearance from the remainder of 
 the plate. 
 
 A question will probably here occur to the 
 reader: What will be the effect if there be not :m 
 oxidating medium present, and the experiment be 
 first performed in a rarefied gas, which possesses 
 no power of chemically acting on the plate ? In 
 this case there will still be a molecular change or 
 disintegration of the plate; the portion of it acted 
 on by the discharge will present a different appear- 
 ance from that which is beyond its reach, and a 
 whitish film, somewhat similar to that seen on the 
 mercurialised portions of a daguerreotype, will gra- 
 dually appear on the portion of the plate affected
 
 ELECTRICITY. 119 
 
 by the discharge. If the gas be a compound, as 
 carbonic oxide, or a mixture, as oxygen and hydro- 
 gen, and consequently contain elements capable 
 of producing oxidation and reduction, then the 
 effect upon the plate will depend upon whether it be 
 positive or negative ; in the former case it will be 
 oxidated, in the latter the oxide, if existing, will be 
 reduced. This effect will also take place in atmo- 
 spheric air, if it be highly rarefied, and can hardly 
 be explained otherwise than by a molecular polaris- 
 ation of the compound gas. If, again, the metal be 
 reduced to a small point, and be of such material 
 that the gas cannot act chemically upon it, it can 
 yet be shown to be disintegrated by the electric 
 spark. Thus, let a fine platinum wire be hermeti- 
 cally sealed in a .glass tube, and the extremity of 
 the tube and the wire ground to a flat surface, so 
 as to expose a section only of the wire; after taking 
 the discharge from this for some time, it will be 
 found that the platinum wire is worn away, and 
 that its termination is sensibly below the level of 
 the glass. If the discharges from such a platinum 
 wire be taken in gas contained in a narrow tube, 
 a cloud or film consisting of a deposit of platinum 
 will be seen on the part of the tube surrounding 
 the point. 
 
 Another curious effect which, in addition to the 
 above, I have detected in the electrical discharge in 
 attenuated media, is that when passing between
 
 120 CORRELATION OF PHYSICAL FORCES. 
 
 terminals of a certain form, as from a wire placed 
 at right angles to a polished plate, the discharge 
 possesses certain phases or fits of an alternate cha- 
 racter, so that, instead of impressing an uniform 
 mark on a polished plate, a series of concentric rings 
 is formed. 
 
 Priestley observed that, after the discharge of a 
 Leyden battery, rings consisting of fused globules 
 of metal were formed on the terminal plates; in 
 my experiments made in attenuated media, al- 
 ternate rings of oxidation and deoxidation are 
 formed. Thus, if the plate be polished, coloured 
 rings of oxide will alternate with rings of polished 
 or unoxidated surface ; and if the plate be pre- 
 viously coated with an uniform film of oxide, the 
 oxide will be removed in concentric spaces, and 
 increased in the alternate ones, showing a lateral 
 alternation of positive and negative electricity, or 
 electricity of opposite character in the same dis- 
 charge. 
 
 It would be hasty to assert that in no case can 
 the electrical disruptive discharge take place with- 
 out the terminals being affected. I have, however, 
 seen no instance of such a result where the dis- 
 charge has been sufficiently prolonged, and the 
 terminals in such a state as could be expected to 
 render manifest slight changes. 
 
 The next question which would occur in follow- 
 ing out the enquiry which has been indicated,
 
 ELECTRICITY. 121 
 
 would probably be, What is the action upon the 
 gas itself ? is this changed in any manner ? 
 
 In answer to this, it must be admitted that, in 
 the present state of experimental knowledge on 
 this subject, certain gases only appear to leave 
 permanent traces of their having been changed by 
 the discharge, while others, if affected by it, which, 
 as will be presently seen, there are reasons to 
 believe they are, return to their normal state im- 
 mediately after the discharge. 
 
 In the former class we may place many com- 
 pound gases, as ammonia, olefiant gas, protoxide 
 of nitrogen, deutoxide of nitrogen, and others. 
 which are decomposed by the action of the dis- 
 charge. Mixed gases are also chemically com- 
 bined: for instance, oxygen and hydrogen unite 
 and form water ; common air gives nitric acid ; 
 chlorine and aqueous vapour give oxygen, the 
 chlorine uniting with the hydrogen of the water. 
 
 But, further than this, in the case of certain 
 elementary gases a permanent change is effected 
 by the electrical discharge. Thus, oxygen, sub- 
 mitted to the discharge is partially changed into 
 the substance called ozone, a substance now con- 
 sidered to be an allotropic condition of oxygen ; 
 and there is reason to believe that, when the 
 change takes place, there is a definite polar con- 
 dition of the gas, and that definite portions of it 
 are affected that in a certain sense one portion
 
 122 CORRELATION OF PHYSICAL FORCES. 
 
 of the oxygen bears temporarily to the other the 
 relation which hydrogen ordinarily does to oxygen. 
 
 If the discharge be passed through the vapour 
 of phosphorus in the vacuum of a good air-pump, 
 a deposit of allotropic phosphorus soon coats the 
 interior of the receiver, showing an analogous 
 change to that produced in oxygen; and in this 
 case a series of transverse bands or stratifications 
 appears in the discharge, showing a most striking 
 alteration in its physical character, dependent on 
 the medium across which it is transmitted. These 
 effects were first observed by me in the year 1852. 
 They have since been much examined by con- 
 tinental philosophers, and much extended by Mr. 
 Gassiot ; but no satisfactory rationale of them has 
 yet been given. 
 
 There are many gases which either do not show 
 any permanent change, or (which is more probably 
 the case) the changes produced in them by the 
 electrical discharge have not yet been detected. 
 Even with these gases, however, the difference of 
 colour, of length, or of the different position of a 
 certain dark space or spaces which appear in the 
 discharge, show that the discharge differs for dif- 
 ferent media. We never find that the discharge 
 has itself added to or subtracted from the total 
 weight of the substances acted on : we find no 
 evidence of a fluid but the visible phenomena 
 themselves ; and those we may account for by the 
 change which takes place in the matter affected.
 
 ELECTRICITY. 123 
 
 I have here, as elsewhere, used words of common 
 acceptation, such as 'matter affected by the dis- 
 charge,' &c., though upon the view I am suggest- 
 ing, the discharge is itself this affection of matter ; 
 and the writing these passages affords, to me at 
 least, a striking instance of how much ideas are 
 bound up in words, when, to express a view differ- 
 ing from the received one, words involving the 
 received one are necessarily used. 
 
 Passing now to the effect of the transmission of 
 electricity by the class of the best conducting 
 bodies, such as the metals and carbon, here, though 
 we cannot at present give the exact character of 
 the motion impressed upon the particles, there are 
 yet many experiments which show that a change 
 takes place in such substances when they are 
 affected by electricity. 
 
 Let discharges from a Leyden jar or battery be 
 passed through a platinum wire, too thick to be 
 fused by the discharges, and free from constraint, 
 it will be found that the wire is shortened ; it has 
 undergone a molecular change, and apparently 
 been acted on by a force transverse to its length. 
 If the discharges be continued, it gradually gathers 
 up in small irregular bends or convolutions. So 
 with voltaic electricity: place a platinum wire in 
 a trough of porcelain, so that when fused it shall 
 retain its position as a wire, and then ignite it by 
 a voltaic battery. As it reaches the point of fusion
 
 124 CORRELATION OF PHYSICAL FORCES. 
 
 it will snap asunder, showing a contraction in 
 length, and consequently a distension or increase 
 in its transverse dimensions. Perform the same 
 experiment with a lead wire, which can be more 
 readily kept in a state of fusion, and follow it, as it 
 contracts, by the terminal wires of the battery ; it 
 will be seen to gather up in nodules, which press on 
 each other like a string of beads of a soft matt-rial 
 which have been longitudinally compressed. 
 
 As we increase the thickness of the wires in 
 these experiments with reference to the electric; il 
 force employed, we lessen the perceptible effect ; 
 but even in this case we shall be enabled safely to 
 infer that some molecular change accompanies the 
 transmission of electricity : the wires are heated in 
 a degree decreasing as their thickness increases 
 but by decreasing the delicacy of our tests as the 
 heating effects decrease in intensity, we may inde- 
 finitely detect the augmentation of temperature 
 accompanying the passage of electricty and 
 wherever there is augmentation of temperature 
 there must be expansion or change of position 
 of the molecules. 
 
 Again, it has been observed that wires which 
 have for a long time transmitted electricity, such 
 as those which have served as conductors for at- 
 mospheric electricity, have their texture changed, 
 and are rendered brittle. In this observation, In >\v- 
 ever, though made by a skilful electrician, M. Pel- 
 tier, the effects of exposure to the atmosphere, to
 
 ELECTRICITY. 125 
 
 changes of temperature, &c., have not been suffi- 
 ciently eliminated to render it worthy of entire 
 confidence. There are, however, other experiments 
 which show that the elasticity of metals is changed 
 by the passage through them of the electric 
 current. 
 
 Thus M. Wertheim has, from an elaborate series 
 of experiments, arrived at the conclusion that there 
 is a temporary diminution in the coefficient of 
 elasticity in wires while they are transmitting the 
 electric current, which is independent of the heat- 
 ing effect of the current. 
 
 M. Dufour has made a considerable number 
 of experiments with the view of ascertaining if 
 any permanent change in metals is effected by elec- 
 trisation. He arrives at the curious result that 
 in a copper wire through which a feeble voltaic 
 current has passed for several days, a notable 
 diminution in tenacity takes place ; while, in an 
 iron wire, the tenacity is increased ; and that these 
 effects were more perceptible when the wires had 
 been electrised for a long time (nineteen days) 
 than for a short time (four days). The copper 
 wire was, in his experiment, not perfectly pure; 
 so that the effect, or a portion of it, might be due 
 to the state of alloy : in the case of iron, the mag- 
 netic character of the metal would probably modify 
 the effects, and might account for the opposite 
 character of the results with these two metals. 
 
 Matteucci has made experiments on the con-
 
 126 CORRELATION OF PHYSICAL FORCES. 
 
 duction of electricity by bismuth in directions 
 parallel or transverse to the planes of principal 
 cleavage, and he finds that bismuth conducts 
 electricity and heat better in the direction of the 
 cleavage planes than in that transverse to them. 
 
 Many other experiments have been made both 
 on the production of thermo-electric currents by 
 two portions of the same crystalline metal, but with 
 the planes of crystallization arranged in different 
 directions relatively to each other, and also on the 
 differences in conduction of heat and electricity 
 according to the direction in which they are 
 transmitted with reference to the planes of crys- 
 tallization. 
 
 It is found, moreover, that the slightest difference 
 in homogeneity in the same metal enables it when 
 heated to produce a thermo-electric current, mid 
 that metals in a state of fusion, in which state they 
 may be presumed to be homogeneous throughout, 
 give no thermo-electric current : thus, hot in contact 
 with cold mercury has been shown by Matteucci to 
 give no thermo-electric current, and the same is the 
 case with portions of fused bismuth unequally heated. 
 
 The fact that the molecular structure or ar- 
 rangement of a body influences indeed I may say 
 determines its conducting power, is by no means 
 explained by the theory of a fluid ; but if electri- 
 city be only a transmission of force or motion, the 
 influence of the molecular state is just what would
 
 ELECTRICITY. 127 
 
 be expected. Carbon, in a transparent crystalline 
 state, as diamond, is as perfect a non-conductor as 
 we know; while in an opaque amorphous state, 
 as graphite or charcoal, it is one of the best con- 
 ductors : thus, in the one state, it transmits light 
 and stops electricity, in the other it transmits elec- 
 tricity and stops light. 
 
 It is a circumstance worthy of remark, that the 
 arrangement of molecules, which renders a solid 
 body capable of transmitting light, is most unfa- 
 vourable to its transmission of electricity, trans- 
 parent solids being very imperfect conductors of 
 electricity ; so all gases readily transmit light, but 
 are amongst the worst conductors of electricity, 
 if indeed, properly speaking, they can be said to 
 conduct at all. 
 
 The conduction of electricity by different classes 
 of bodies has been generally regarded as a question 
 of degree : thus metals were viewed as perfect con- 
 ductors, charcoal less so, water and other liquids 
 as imperfect conductors, &c. But, in fact, though 
 between one metal and another the mode of trans- 
 mission may be the same and the difference one of 
 degree, a different molecular effect obtains, when 
 we contrast metals with electrolytic liquids and 
 these with gases. 
 
 Attenuated gases may be, in one sense, regarded 
 as non-conductors, in another as conductors ; thus 
 if gold leaves be made to diverge, by electrical
 
 128 CORRELATION OF PHYSICAL FORCES. 
 
 repulsion, in air at ordinary pressure, they in a 
 short time collapse; while in highly-rarefied air, 
 or what is commonly termed a vacuum, they 
 remain divergent for days ; and yet electricity of 
 a certain degree of tension passes readily across 
 attenuated air, and with difficulty across air of 
 ordinary density. 
 
 Again, where the electrical terminals are brought 
 to a state of visible ignition, there are symptoms of 
 the transmission of electricity of low tension across 
 gases; but no such effects have been detected at 
 lower temperatures. All this presents a strong 
 argument in favour of the transmission of electri- 
 city across gases being effected by the disruptive 
 discharge, and not by a conduction similar to that 
 which takes place with metals or with electrolytes. 
 
 The ordinary attractions and repulsions of elec- 
 trified bodies present no more difficulty when re- 
 garded as being produced by a change in the state 
 or relations of the matter affected, than do the 
 attractions of the earth by the sun, or of a leaden 
 ball by the earth ; the hypothesis of a fluid is not 
 considered necessary for the latter, and need not 
 be so for the former class of phenomena. How the 
 phenomena are produced to which the term at- 
 traction is applied is still a mystery. Newton, 
 speaking of it, says, ' What I call attraction may 
 be performed by impulse, or by some other means 
 unknown to me. I use that word here to signify
 
 ELECTRICITY. 129 
 
 only in general any force by which bodies tend to- 
 wards one another, whatsoever be the cause." If 
 we suppose a fluid to act in attractions and repul- 
 sions, the imponderable fluid must drag or push 
 the matter with it : thus when we feel a stream of 
 air rushing from an electrified metallic point, each 
 molecule of air contiguous to the point being 
 repelled, another takes its place, which is in its turn 
 repelled; how does a hypothetic fluid assist us 
 here? If we say the electrical fluid repels itself, 
 or the same electricity repels itself, we must go 
 farther and assert, that it not only repels itself, 
 but either communicates its repulsive force to the 
 particles of the air, or carries with it the particle 
 of air in its passage. Is it not more easy to 
 assume that the particle of air is in such a state 
 . that the ordinary forces which keep it in equilibrium 
 I are disturbed by the electrical force, or force in a 
 ! definite direction communicated to it, and that 
 i thus each particle in turn recedes from the point ? 
 i As this latter force is increased, not only does the 
 I particle of air which was contiguous to the metallic 
 point recede, but the cohesion of the extreme par- 
 ticles of metal may be overcome to such an extent 
 that these are detached, and the brush or spark 
 may consist wholly or in part of minute particles 
 of the metal itself thrown off. Of this there is 
 some evidence, though the point can hardly be con- 
 sidered as proved. A similar effect undoubtedly
 
 130 CORRELATION OF PHYSICAL FORCES. 
 
 takes place with voltaic electricity, acting upon a ter- 
 minal immersed in a liquid ; thus if metallic terminals 
 of a powerful voltaic battery be immersed in water, 
 metal, or the oxide of metal, is forcibly detached, 
 producing great heat at the point of disruption. 
 
 If we apply ourselves to the effect of electricity 
 in the animal economy, we find that the first 
 rationale given of the convulsive effect produced 
 by transmission through the living or recently 
 killed animal was, that electricity itself, something 
 substantive, passed rapidly through the body, and 
 gave rise to the contractions; step by step we 
 are now arriving at the conviction that conse- 
 cutive particles of the nerves and muscles are af- 
 fected. Thus the contractions which the prepared 
 leg of a frog undergoes at the moment it is sub- 
 mitted to a voltaic current, cease after a time if 
 the current be continued, and are renewed on 
 breaking the circuit, i. e. at the moment when the 
 current ceases to traverse it. The excitability of a 
 nerve, moreover, or its power of producing muscular 
 contraction, is weakened or destroyed by the trans- 
 mission of electricity in one direction, while the 
 excitability is increased by the transmission of 
 electricity in the opposite direction; showing that 
 the fibre or matter itself of the nerve is changed 
 by electrisation, and changed in a manner bearing 
 a direct relation to the other effects produced by 
 electricity.
 
 ELECTRICITY. 131 
 
 Portions of muscle and of nerve present different 
 electrical states with reference to other portions of 
 the same muscle or nerve ; thus the external part 
 of a muscle bears the same relation to the internal 
 part as platinum does to zinc in the voltaic battery ; 
 and delicate galvanoscopes will show electrical 
 effects when interposed in a conducting circuit 
 connecting the surface of a nerve with its interior 
 portions. Matteucci has proved that a species of 
 voltaic pile may be formed by a series of slices of 
 muscle, so arranged that the external part of one 
 slice may touch the internal part of the next, and 
 so on. 
 
 Lastly, the magnetic effects produced by electri- 
 city also show a change in the molecular state of 
 the magnetic substance affected ; as we shall see 
 when the subject of magnetism is discussed. 
 
 I have taken in succession all the known classes 
 of electrical phenomena ; and, as far as I am aware, 
 there is not a single electrical effect, where, if a 
 close investigation be instituted, arid the materials 
 chosen in a state for exhibiting minute changes, 
 evidence of molecular change will not be detected ; 
 so that, excepting those cases where innmtesimally 
 small quantities of matter are acted on, and our 
 means of detection fail, electrical effects are known 
 to us only as changes of ordinary matter. It seems 
 to me as easy to imagine these changes to be 
 effected by a force acting in definite directions, as 
 
 K 2
 
 132 CORRELATION OF PHYSICAL FORCES. 
 
 by a fluid which has no independent or sensible 
 existence, and which, it must be assumed, is as- 
 sociated with, or exerts a force acting upon or 
 dinary matter, or matter of a different order from 
 the supposed fluid. As the idea of the hypothetic 
 fluid is pursued, it gradually vanishes, and resolves 
 itself into the idea of force. The hypothesis of 
 matter without weight presents in itself, as I be- 
 lieve, fatal objections to the theories of electrical 
 fluids, which are entirely removed by viewing 
 electricity as force, and not as matter. 
 
 If it be said that the effects we have been consi- 
 dering may still be produced by a fluid, and that 
 this fluid acts upon ordinary matter in certain 
 cases, polarising the matter affected or arranging 
 its particles in a definite direction, whilst in others, 
 by its attractive or repulsive force, it carries with 
 it portions of matter ; then, if the fluid in itself be 
 incapable of recognition by any test, if it be only 
 evidenced by the changes which it operates in pon- 
 derable matter, the words fluid and force become 
 identical in meaning ; we may as well say that the 
 attraction of gravitation or weight is occasioned 
 by a fluid, as that electrical changes are so. 
 
 When, as is constantly done in common parlance, 
 a house is said to be struck, windows broken, metals 
 fused or dissipated by the electrical fluid, are not 
 the expressions used such as, if not sanctioned by 
 habit, would seem absurd? In all the cases of in-
 
 ELECTRICITY. 133 
 
 jury done by lightning there is no fluid perceptible ; 
 the so-called sulphurous odour is either ozone de- 
 veloped by the action of electricity on atmospheric 
 air, or the vapour of some substance dissipated 
 by the discharge ; does it not then seem more 
 consonant with experience to regard these effects 
 as produced by force, as we have analogous effects 
 produced by admitted forces, in cases where no one 
 would invoke the aid of a hypothetic fluid for ex- 
 planation. For instance, glasses may be broken 
 by electrical discharges ; so may they by sonorous 
 vibrations. Metals electrified or magnetised will 
 emit a sound ; so they will if struck, or if a musical 
 note with which they can vibrate in unison be 
 sounded near to them. 
 
 Even chemical decomposition, in cases of feeble 
 affinity, may be produced by purely mechanical 
 effects. A number of instances of this have been 
 collected by M. Becquerel ; and substances whose 
 constituents are held together by feeble affinities, 
 such as iodide of nitrogen and similar compounds, 
 are decomposed by the vibration occasioned by sound. 
 
 If, instead of being regarded as a fluid or 
 imponderable matter sui generis, electricity be re- 
 garded as the motion of an ether, equal difficulties 
 are encountered. Assuming ether to pervade the 
 pores of all bodies, is the ether a conductor or non- 
 conductor ? If the latter that is, if the ether be 
 incapable of transmitting the electrical wave the
 
 134 CORRELATION OF PHYSICAL FORCES. 
 
 ethereal hypothesis of electricity necessarily fails ; 
 but if the motion of the ether constitute what we 
 call conduction of electricity, then the more porous 
 bodies, or those most permeable by the ether, should 
 be the best conductors. But this is not the case. 
 If, again, the metal and the air surrounding it are 
 both pervaded by ether, why should the electrical 
 wave aifect the ether in the metal, and not stir 
 that in the gas? To support an ethereal hypo- 
 thesis of electricity, many additional and hardly 
 reconcilable hypotheses must be imported. 
 
 The fracture and comminution of a non-con- 
 ducting body, the fusion or dispersion of a metallic 
 wire by the electrical discharge, are effects equally 
 difficult to conceive upon the hypothesis of an 
 ethereal vibration, as upon that of a fluid, but are 
 necessary results of the sudden subversion of mole- 
 cular polarisation, or of a sudden or irregular 
 vibratory movement of the matter itself. We see 
 similar effects produced by sonorous vibrations, 
 which might be called conduction and non-con- 
 duction of sound. One body transmits sound 
 easily, another stops or deadens it, as it is termed 
 i.e. disperses the vibrations, instead of continuing 
 them in the same direction as the primary impulse ; 
 and solid bodies may, as has been above observed, 
 be shivered by sudden impulses of sound in those 
 cases where all the parts of the body cannot uni- 
 formly carry on the undulatory motion.
 
 ELECTRICITY. 135 
 
 The progressive stages in the History of Phy- 
 sical Philosophy will account in a great measure 
 for the adoption by the early electricians of the 
 theories of fluids. 
 
 The ancients, when they witnessed a natural 
 phenomenon, removed from ordinary analogies, 
 and unexplained by any mechanical action known 
 to them, referred it to a soul, a spiritual or pre- 
 ternatural power : thus amber and the magnet 
 were supposed by Thales to have a soul ; the 
 functions of digestion, assimilation, &c., were 
 supposed by Paracelsus to be effected by a spirit 
 (the Archaeus). Air and gases were also at 
 first deemed spiritual, but subsequently became 
 invested with a more material character ; and the 
 word gas, from geist, a ghost or spirit, affords us an 
 instance of the gradual transmission of a spiritual 
 into a physical conception. 
 
 The establishment by Torricelli of the ponder- 
 able character of air and gas, showed that sub- 
 stances which had been deemed spiritual and 
 essentially different from ponderable matter were 
 possessed of its attributes. A less superstitious 
 mode of reasoning ensued, and now aeriform fluids 
 were shown to be analogous in many of their 
 actions to liquids or known fluids. A belief in the 
 existence of other fluids, differing from air as this 
 differed from water, grew up, and when a new 
 phenomenon presented itself, recourse was had to
 
 136 CORRELATION OF PHYSICAL FORCES. 
 
 a hypothetic fluid for explaining the phenomenon 
 and connecting it with others ; the mind once 
 possessed of the idea of a fluid, soon invested it 
 with the necessary powers and properties, and 
 grafted upon it a luxurious vegetation of imaginary 
 offshoots. 
 
 In what I am here throwing out, I wish to 
 guard myself from being supposed to state that 
 the course of theory, historically viewed, followed 
 exactly the dates of the discoveries which were 
 effectual in changing its character; sometimes a 
 discovery precedes, at other times it succeeds a 
 change in the general course of thought; sometimes, 
 and perhaps most frequently, it does both i. e. the 
 discovery is the result of a tendency of the :i ;_; 
 and of the continually improved methods of 
 observation, and when made, it strengthens smd 
 extends the views which have led to it. I think 
 the phases of thought which physical philosophers 
 have gone through, will be found generally such as 
 I have indicated, and that the gradual accumula- 
 tion of discoveries which has taken place during 
 the more recent periods, by showing what effects 
 can be produced by dynamical causes alone , is 
 rapidly tending to a general dynamical theory into 
 which that of the imponderable fluids prom i si s 
 ultimately to merge. 
 
 Commencing with electricity as an initiating 
 force, we get motion directly produced by it in 

 
 
 ELECTRICITY. 137 
 
 various forms ; for instance, in the attraction and 
 repulsion of bodies, evidenced by mobile electro- 
 meters, such as that of Cuthbertson, where large 
 masses are acted on ; the rotation of the fly-wheel, 
 another form of electrical repulsion, and the 
 deflection of the galvanometer needle, are also 
 modes of palpable, visible motion. 
 
 It would follow, from the reasoning in this essay, 
 that when electricity performs any mechanical 
 work which does not return to the machine, elec- 
 trical power is lost. It would be unsuitable to the 
 scope of this work to give the mathematical 
 labours of M. Clausius and others here ; but the 
 following experiment, which I devised for making 
 the result evident to an audience at the Royal In- 
 stitution, will form a useful illustration : A Leyden 
 jar, of one square foot coated surface, has its in- 
 terior connected with a Cuthbertson's electrometer, 
 between which and the outer coating of the jar 
 are a pair of discharging balls fixed at a certain 
 distance (about half an inch apart). Between the 
 Leyden jar and the prime conductor is inserted a 
 small unit jar of nine inches surface, the knobs of 
 which are 0'2 inch apart. 
 
 The balance of the electrometer is now fixed by 
 a stiff wire inserted between the attracting knobs, 
 and the Leyden jar charged by discharges from the 
 unit jar. After a certain number of these, say 
 twenty, the discharge of the large jar takes place
 
 138 COKKELATION OF PHYSICAL FORCES. 
 
 across the half-inch interval. This may be viewed as 
 the expression of electrical power received from the 
 unit jar. The experiment is now repeated, the wire 
 between the balls having been removed, and there- 
 fore the ' tip,' or the raising of the weight, is per- 
 formed by the electrical repulsion and attraction of 
 the two pairs of balls. At twenty discharges of the 
 unit jar the balance is subverted, and one attracting 
 knob drops upon the other ; but no discharge takes 
 place, showing that some electricity has been lost or 
 converted into the mechanical power which raised 
 the balance. 
 
 By another mode of expression, the electricity 
 may be supposed to be masked or analogous to 
 latent heat, and it would be restored if the ball wore 
 brought back without discharge by extraneous 
 force. If the discharge or other electrical effects 
 were the same in both cases, then, since the raising 
 of the ball or weight is an extra mechanical effort, 
 and since the weight is capable by its fall of pro- 
 ducing electricity, heat, or other force, it would 
 seem that force could be got out of nothing, or 
 perpetual motion obtained. 
 
 The above experiment is suggestive of others of 
 a similar character, which may be indefinitely 
 varied. Thus I have found that two balls made 
 to diverge by electricity do not give to an electro- 
 meter the same amount of electricity as they do 
 if, whilst similarly electrified, they are kept forcibly
 
 
 ELECTRICITY. 139 
 
 together. This experiment is the converse of the 
 former one. There is an advantage in electrical 
 experiments of this class as compared with those on 
 heat, viz. that though there is no perfect insulation 
 for electricity, yet our means of insulating it are 
 immeasurably superior to any attainable for heat. 
 
 Electricity directly produces heat, as shown in 
 the ignited wire, the electric spark, and the voltaic 
 arc : in the latter the most intense heat with which 
 we are acquainted so intense, indeed, that it 
 cannot be measured, as every sort of matter is 
 dissipated by it. 
 
 In the phenomenon of electrical ignition, as 
 shown by a heated conjunctive wire, the relation of 
 force and resistance, and the correlative character 
 of the two forces, electricity and heat, are strikingly 
 demonstrated. Let a thin wire of platinum join 
 the terminals of a voltaic battery of suitable 
 power, the wire will be ignited, and a certain 
 amount of chemical action will take place in the 
 cells of the battery a definite quantity of zinc 
 being dissolved and of hydrogen eliminated in a 
 given time. If now the platinum wire be im- 
 mersed in water, the heat will, from the circulating 
 currents of the liquid, be more rapidly dissipated, 
 and we shall instantly find that the chemical 
 action in the battery will be increased, more zinc 
 will be dissolved, and more hydrogen eliminated 
 for the same tune ; the heat being conveyed away
 
 140 CORRELATION OF PHYSICAL FORCES. 
 
 by the water, more chemical action is required to 
 generate it, just as more fuel is required in pro- 
 portion as evaporation is more rapid. 
 
 Keverse the experiment, and instead of placing 
 the wire in water, place it in the flame of a spirit 
 lamp, so that the force of heat meets with greater 
 resistance to its dissipation. We now find that 
 the chemical action is less than in the first or 
 normal experiment. If the wire be placed in other 
 different gaseous or liquid media, we shall find that 
 the chemical action of the battery will be propor- 
 tioned to the facility with which the heat is cir- 
 culated or radiated by these media, and we thus 
 establish an alternating reciprocity of action be- 
 tween these two forces : a similar reciprocity may 
 be established between electricity and motion, mag- 
 netism and motion, and so of other forces. If it 
 cannot be realised with all, it is probably because 
 we have not yet eliminated interfering actions. If 
 we carefully think over the matter, we shall, unless 
 I am much mistaken, arrive at the conclusion that 
 it cannot be otherwise, unless it be supposed that a 
 force can arise from nothing can exist without 
 antecedent force. 
 
 In the phenomenon of the voltaic arc, the electric 
 spark, &c., to which I have already adverted, electri- 
 city directly produces light of the greatest intensity 
 of any artificially obtained. It directly produces 
 magnetism, as shown by Oersted, who first distinctly
 
 ELECTRICITY. 141 
 
 proved the connection between electricity and mag- 
 netism. These two forces act upon each other, not 
 in straight lines, as all other known forces do, but in 
 a rectangular direction ; that is, bodies affected by 
 dynamic electricity, or the conduits of an electric 
 current, tend to place magnets at right angles to 
 them; and, conversely, magnets tend to place 
 bodies conducting electricity at right angles to 
 them. Thus an electric current appears to have a 
 magnetic action, in a direction cutting its own at 
 right angles ; or, supposing its section to be a 
 circle, tangential to it: if, then, we reverse the 
 position, and make the electric current form a 
 series of tangents to an imaginary cylinder, this 
 cylinder should be a magnet. This is effected in 
 practice by coiling, a wire as a helix or spiral, and 
 this, when conducting an electrical current, is to 
 all intents and purposes a magnet. A soft iron 
 core placed within such a helix has the property 
 of concentrating its power, and then we can, by 
 connection or disconnection with the source of 
 electricity, instantly make or unmake a most 
 powerful magnet. 
 
 We may figure to the mind electrified and mag- 
 netised matter, as lines of which the extremities 
 repel each other in a definite direction ; thus, if a 
 line A B represent a wire affected by electricity, and 
 superposed on c D a wire affected by magnetism, 
 the extreme points A and B will be repelled to the
 
 142 CORRELATION OF PHYSICAL FORCES. 
 
 farthest distances from the points c and D, and the 
 line A B be at right angles to the line c D ; and so, if 
 the lines be subdivided to any extent, each will have 
 two extremities or poles repulsive of those of the 
 other. If the line of matter affected by electricity 
 be a liquid, and consequently have entire mobility 
 of particles, a continuous movement will be pro- 
 duced by magnetisation, each particle successively 
 tending, as it were, to fly off at a tangent from the 
 magnet: thus, place a flat dish containing acidu- 
 lated water on the poles of a powerful magiii-t, 
 immerse the terminals of a voltaic battery in the 
 liquid just above the magnetic poles, so that the 
 lines of electricity and of magnetism coincide ; the 
 water will now assume a movement at right angles 
 to this line, flowing continuously, as if blown by an 
 equatorial wind, which may be made east or west 
 with reference to the magnetic poles by altering 
 the direction of the electrical current: a similar 
 effect may be produced with mercury. These 
 cases afford an additional argument to those pre- 
 viously mentioned, of the particles of matter being 
 affected by the forces of electricity and magnetism 
 in a way irreconcilable with the fluid or ethereal 
 hypothesis. 
 
 The representation of transverse direction by 
 magnetism and electricity appears to have led 
 Coleridge to parallel it by the transverse expan- 
 sion of matter, or length and breadth, though he
 
 ELECTKICITY. 143 
 
 injured the parallel by adding galvanism as depth : 
 whether a third force exists which may bear this 
 relation to electricity and magnetism is a question 
 upon which we have no evidence. 
 
 The ratio which the attractive magnetic force 
 produced bears to the electric current producing it 
 has been investigated by many experimentalists and 
 mathematicians. The data are so numerous and 
 so variable, that it is difficult to arrive at definite 
 results. Thus the relative size of the coil and the 
 iron, the temper or degree of hardness of the latter, 
 its shape, or the proportion of length to diameter, 
 the number of coils surrounding it, the conducting 
 power of the metal of which the coils are formed, 
 the size of the keeper or iron in which magnetism 
 is induced, the degree of constancy of the battery, 
 &c., complicate the experiments. 
 
 The most trustworthy general relation which 
 has been ascertained is, that the magnetic attrac- 
 tion is as the square of the electric force ; a result 
 due to the researches of Lenz and Jacobi, and also 
 of Sir W. S. Harris. 
 
 Lastly, electricity produces chemical affinity ; and 
 by its agency we are enabled to obtain effects of 
 analysis or synthesis with which ordinary che- 
 mistry does not furnish us. Of these effects we have 
 examples in the brilliant discoveries, by Davy, of 
 the alkaline metals, and in the peculiar crystalline 
 compounds made known by Crosse and Becquerel.
 
 144 CORRELATION OF PHYSICAL FORCES. 
 
 LIGHT. 
 
 IN entering on the subject of LIGHT, it will be well 
 to describe briefly, and in a manner as far as may 
 be independent of theory, the effects to which the 
 term polarisation has been applied. 
 
 When light is reflected from the surface of water, 
 glass, or many other media, it undergoes a change 
 which disables it from being again similarly re- 
 flected in a direction at right angles to that at 
 which it has been originally reflected. Light so 
 affected is said to be polarised ; it will always be 
 capable of being reflected in planes parallel to the 
 plane hi which it has been first reflected, but in- 
 capable of being reflected in planes at right angles 
 to that plane. At planes having a direction inter- 
 mediate between the original plane of reflection, and 
 a plane at right angles to it, the light will be capable 
 of being partially reflected, and more or less so 
 according as the direction of the second plane of 
 reflection is more or less coincident with the ori- 
 ginal plane. Light, again, when passed through a 
 crystal of Iceland spar, is what is termed doubly 
 refracted, i.e. split into two divisions or beams, 

 
 LIGHT. 145 
 
 each having half the luminosity of the original 
 incident light ; each of these beams is polarised in 
 planes at right angles to each other; and if they 
 be intercepted by the mineral tourmaline, one of 
 them is absorbed, so that only one polarised beam 
 emerges. Similar effects may be produced by 
 certain other reflections or refractions. A ray of 
 light once polarised in a certain plane continues so 
 affected throughout its whole subsequent course; 
 and at any indefinite distance from the point where 
 it originally underwent the change, the direction 
 of the plane will be the same, provided the media 
 through which it is transmitted be air, water, or 
 certain other transparent substances which need 
 not be enumerated. If, however, the polarised ray, 
 instead of passing through water, be made to pass 
 through oil of turpentine, the definite direction in 
 which it is polarised will be found to be changed; 
 and the change of direction will be greater accord- 
 ug to the length of the column of interposed liquid, 
 tjlnstead of being an uniform plane, it will have a 
 'Jcurvilinear direction, similar to that which a strip 
 ijof card would have if forced along two opposite 
 grooves of a rifle-barrel. This curious effect is 
 produced in different degrees by different media. 
 The direction also varies; the rotation, as it is 
 termed, being sometimes to the right hand and 
 sometimes to the left, according to the peculiar
 
 146 CORRELATION OF PHYSICAL FORCES. 
 
 molecular character of the medium through which 
 the polarised ray is transmitted. These effects will 
 be presently reverted to. 
 
 Light is, perhaps, that mode of force the reci- 
 procal relations of which with the others have been 
 the least traced out. Until the discoveries of 
 Niepce, Daguerre, and Talbot, very little could be 
 definitely predicated of the action of light in pro- 
 ducing other modes of force. Certain chemical 
 compounds, among which stand pre-eminent the 
 salts of silver, have the property of suffering de- 
 composition when exposed to light. If, for in- 
 stance, recently formed cliloride of silver be sub- 
 mitted to luminous rays, a partial decomposition 
 ensues ; the chlorine is separated and expelled by 
 the action of light, and the silver is precipitated. 
 By this decomposition the colour of the substance 
 changes from white to blue. If now, paper be 
 impregnated with chloride of silver, which can be 
 done by a simple chemical process, then partially 
 covered with an opaque substance, a leaf for ex- 
 ample, and exposed to a strong light, the cliloride 
 will be decomposed in all those parts of the paper 
 where the light is not intercepted, and we shall 
 have, by the action of light, a white image of the 
 leaf on a purple ground. If similar paper be placed 
 in the focus of a lens in a camera-obscura, the 
 objects there depicted will decompose the chloride, 
 just in the proportion in which they are luminous;
 
 LIGHT. 147 
 
 and thus, as the most luminous parts of the image 
 will most darken the chloride, we shall have a 
 picture of the objects with reversed lights and 
 shadows. The picture thus produced would not 
 be permanent, as subsequent exposure would 
 darken the light portion of the picture : to fix it, 
 the paper must be immersed in a solution which 
 has the property of dissolving chloride of silver, 
 but not metallic silver. Iodide of potassium will 
 effect this ; and the paper being washed and dried 
 will then preserve a permanent image of the de- 
 j picted objects. This was the first and simple pro- 
 cess of Mr. Talbot; but it is defective as to the 
 purposes aimed at, in many points. First, it is 
 not sufficiently sensitive, requiring a strong light 
 and a long time to produce an image; secondly, 
 the lights and shadows are reversed ; and thirdly, 
 the coarse structure of the finest paper does not 
 admit of the delicate traces of objects being dis- 
 tinctly impressed. These defects have been to a 
 great extent remedied by a process subsequently 
 discovered by Mr. Talbot, and which bears his 
 name, and which has led to the collodion process, 
 and others unnecessary to be detailed here. 
 
 The photographs of M. Daguerre, with which all 
 are now familiar, are produced by holding a plate 
 of highly-polished silver over iodine. A thin film of 
 iodide of silver is thus formed on the surface of the 
 metal; and when these iodized plates are exposed 
 
 L 2
 
 148 CORRELATION OF PHYSICAL FORCES. 
 
 in the camera, a chemical alteration takes place. 
 The portions of the plate on which the light has 
 impinged part with some of the iodine, or are 
 otherwise changed for the theory is somewhat 
 doubtful so as to be capable of ready amalgama- 
 tion. When, therefore, the plate is placed over the 
 vapour of heated mercury, the mercury attaches 
 itself to the portions affected by light, and gives 
 them a white frosted appearance ; the intermediate m 
 tints are less affected, and those parts where no 
 light has fallen, by retaining their original polish, | 
 appear dark: the iodide of silver is then washed 
 off by hyposulphite of soda, which has the property -J 
 of dissolving it, and there remains a picture mm 
 which the lights and shadows are as in nature, and 
 the molecular uniformity of the metallic surface 
 enables the most microscopic details to be depicted J 
 with perfect accuracy. By using chloride of 
 iodine, or bromide of iodine, instead of iodine, the 
 equilibrium of chemical forces is rendered still more 
 unstable, so that images may betaken in an inde- 
 finitely short period a period practically instan-M 
 taneous. 
 
 It would be foreign to the object of this essajH 
 to enter upon the many beautiful details into 
 which the science of photography has branched 
 out, and the many valuable discoveries and prac- 
 tical applications to which it has led. The short 
 statement I have given above is perhaps
 
 LIGHT. 149 
 
 fluous, as, though they were new and surprising 
 at the period when these Lectures were delivered, 
 photographic processes have now become familiar, 
 not only to the cultivator of science, but to the 
 artist and amateur: the important point for con- 
 sideration here is that light will chemically or 
 molecularly affect matter. Not only will the par- 
 ticular compounds above selected as instances be 
 changed by the action of light ; but a vast number 
 of substances, both elementary and compound, are 
 notably affected by this agent, even those appa- 
 rently the most unalterable in character, such as 
 metals: so numerous, indeed, are the substances 
 affected, that it has been supposed, not without 
 reason, that matter of every description is altered 
 by exposure to light. 
 
 The permanent impression stamped on the 
 molecules of matter by light can be made to re- 
 peat itself by the same agency, but always with 
 decreasing force. Thus a photograph placed op- 
 posite a camera containing a sensitive plate will 
 be reproduced, but if the size of the image be equal 
 to the picture, the second picture will be fainter 
 than the first, and so on. Thus again, a photograph 
 taken on a dull day cannot, by being placed in bright 
 sunshine, be made to reproduce a second photograph 
 of the same size and more distinctly marked than 
 itself; I at least have never succeeded in such re- 
 production, and I am not aware that others have :
 
 150 CORRELATION OF PHYSICAL FORCES. 
 
 the image loses in intensity as light itself does by 
 each transmission. The surface of the metal or 
 paper may give a brighter image from its being 
 exposed to a more intense light, but the photo- 
 graphic details are limited to the intensity of the 
 first impression, or rather to something short of 
 tin's. A question of theoretical interest arises 
 from the consideration of these reproduced photo- 
 graphs. We know that the luminosity of the image 
 at the focus of a telescope is limited by the arm of 
 the object-glass. The image of any given object can- 
 not be intensified by throwing upon it extraneous 
 light ; it is indeed diminished in intensity, and when 
 for certain purposes astronomers illuminate tin- 
 fields of their telescopes, they are obliged to be con- 
 tented with a loss of intensity in the telescopic image. 
 
 Now, let us suppose that the minutest details 
 in the image of an object seen in a given telescope, 
 and with a given power, are noted; that then a 
 photographic plate is placed in the focus of the 
 same telescope so as to obtain a permanent impres- 
 sion of the image which has been viewed by the 
 eye-glass. Could the observer, by throwing a 
 beam of condensed light upon the photograph, en- 
 able himself to bring out fresh details? or in other 
 words, could he use with advantage a higher power 
 applied to the illuminated photograph? 
 
 It is, perhaps, hardly safe to answer a priori this 
 question; but the experiment of reproducing photo-
 
 LIGHT. 151 
 
 graphs would seem to show that more than the 
 initial light cannot be got, and that we cannot 
 expect to increase telescopic power by photography, 
 though we may render observations more conve- 
 nient, may by its means fix images seen on rare and 
 favourable occasions, and may preserve permanent 
 and infallible records of the past state of astro- 
 nomical objects. 
 
 The effect of light on chemical compounds affords 
 us a striking instance of the extent to which a 
 force, ever active, may be ignored through succes- 
 sive ages of philosophy. If we suppose the walls 
 of a large room covered with photographic appa- 
 ratus, the small amount of light reflected from the 
 face of a person situated in its centre would simul- 
 taneously imprint his portrait on a multitude of 
 recipient surfaces. Were the cameras absent, but 
 the room coated with photographic paper, a change 
 would equally take place in every portion of it, 
 though not a reproduction of form and figure. 
 As other substances not commonly called photo- 
 graphic are known to be affected by light, the list 
 of which might be indefinitely extended, it becomes 
 a curious object of contemplation to consider how 
 far light is daily operating changes in ponderable 
 matter how far a force, for a long time recog- 
 nised only in its visual effects, may be constantly 
 producing changes in the earth and atmosphere, in 
 addition to the changes it produces in organised
 
 151 CORRELATION OF PHYSICAL FORCES. 
 
 structures which are now beginning to be exten- 
 sively studied. Every portion of light may be 
 supposed to write its own history by a change 
 more or less permanent in ponderable matter. 
 
 The late Mr. George Stephenson had a favourite 
 idea, which would now be recognised as more phi- 
 losophical than it was in his day, viz. that the 
 light, which we nightly obtain from coal or other 
 fuel, was a reproduction of that which had at one 
 time been absorbed by vegetable structures from 
 the sun. The conviction that the transient gleam 
 leaves its permanent impress on the world's history, 
 also leads the mind to ponder over the many pos- 
 sible agencies of which we of the present day may 
 be as ignorant as the ancients were of the chemical 
 action of light. 
 
 I have used the term light, and affected by light, 
 in speaking of photographic effects ; but, though 
 the phenomena derived their name from light, it 
 has been doubted by many competent investigators 
 whether the phenomena of photography are not 
 mainly dependent upon a separate agent accom- 
 panying light, rather than upon light itself. It is, 
 indeed, difficult not to believe that a picture, taken 
 in the focus of a camera-obscura, and which ropiv- 
 sents to the eye all the gradations of light and 
 shade shown by the original luminous image, is not 
 an effect of light ; certain it is, however, that ihc 
 different coloured rays exercise different actions
 
 LIGHT. 153 
 
 upon various chemical compounds, and that the 
 effects on many, perhaps on all of them, are not 
 proportionate in intensity to the effects upon the 
 visual organs. Those effects, however, appear to 
 be more of degree than of specific difference ; and, 
 without pronouncing myself positively upon the 
 question, hitherto so little examined, I think it will 
 be safer at present to regard the action on photo- 
 graphic compounds as resulting from a function of 
 light. So viewing it, we get light as an initiating 
 force, capable of producing, mediately or imme- 
 diately, the other modes of force. Thus, it imme- 
 diately produces chemical action ; and having this, 
 we at once acquire a means of producing the others. 
 At my Lectures in 1843, I showed an experiment 
 by which the production of all the other modes 
 of force by light is exhibited : I may here shortly 
 describe it. A prepared daguerreotype plate is 
 enclosed in a box filled with water, having a glass 
 front with a shutter over it. Between this glass 
 and the plate is a gridiron of silver wire ; the plate 
 is connected with one extremity of a galvanometer 
 coil, and the gridiron of wire with one extremity of 
 a Breguet's helix an elegant instrument, formed 
 by a coil of two metals, the unequal expansion of 
 which indicates slight changes in temperature 
 the other extremities of the galvanometer and 
 helix are connected by a wire, and the needles 
 brought to zero. As soon as a beam of either 

 
 154 CORRELATION OF PHYSICAL FORCES. 
 
 daylight or the oxyhydrogen light is, by raising 
 the shutter, permitted to impinge upon the plate, 
 the needles are deflected. Thus light being the 
 initiating force, we get chemical action on the plate, 
 electricity circulating through the wires, magnetism 
 in the coil, heat in the helix, and motion in the 
 needles. 
 
 If two plates of platinum be placed in acidulated 
 water, and connected with a delicate galvanometer, 
 the needle of this is always deflected, a result due 
 to films of gas or other matter on the surface of 
 the platinum, which no cleaning can remove. If, 
 after the needle has returned to zero, which will 
 not be the case for some hours or even days, one 
 of the platinum surfaces be exposed to light, a 
 fresh deflection of the needle takes place, due, as 
 far as I have been able to resolve it, to an 
 augmentation of the chemical action which had 
 occasioned the orginal deflection, for the deviation 
 is in the same direction. If, instead of white light, 
 coloured light be permitted to impinge on the 
 plate, the deviation is greater with blue than with 
 red or yellow light, showing, in addition to other 
 tests, that the effect is not due to the heat of the 
 sun's rays, as the calorific effects of light are 
 greater with red than with blue light, while the 
 chemical effects are the inverse. 
 
 There are other apparently more direct agencies 
 of light in producing electricity and magnetismj
 
 LIGHT. 155 
 
 such as those observed by Morichini and others, as 
 well as its effects upon crystallization ; but these 
 results have hitherto been of so indefinite a cha- 
 racter, that they can only be regarded as present- 
 ing fields for experiment, and not as proving the 
 relations of light to the other forces. 
 
 Light would seem directly to produce heat in 
 the phenomena of what is termed absorption of 
 light : in some of these we find that heat is deve- 
 loped in proportion to the disappearance of light. 
 It was thought, and Franklin's experiment of 
 different coloured cloths on snow exposed to the 
 sun seemed to prove it, that heat was produced in 
 a direct ratio to the absorption of light, but Dr. 
 Tyndall has shown that this result does not depend 
 on the colour, but. on the chemical and physical 
 character of the substance exposed, and that the 
 heating effect is due mainly to non-luminous radia- 
 tion. The heating powers of different colours are 
 by no means in exact proportion to the intensity of 
 their light as affecting the visual organs. Thus 
 red light, when produced by refraction from a 
 prism of glass, produces greater heating effect than 
 yellow light in the phenomena of absorption, as has 
 been observed by Sir W. Herschel. The red rays 
 appear, however, to produce a dynamic effect 
 greater than any of the others ; thus they pene- 
 trate water to a greater depth than the other 
 colours; but, according to Dr. Seebeck, we get a
 
 156 CORRELATION OF PHYSICAL FORCES. 
 
 further anomaly, viz. that when light is refracted 
 by a prism of water the yellow rays produce the 
 greater heating effect. The subject, therefore, 
 requires much more experiment before we can 
 ascertain the rationale of the action of the forces 
 of light and heat in this class of phenomena. 
 
 In a former edition of this Essay, I suggested 
 the following experiment on this subject: Let a 
 beam of light be passed through two plates of 
 tourmaline, or similar substance, and the tempera- 
 ture of the second plate, or that on which the light 
 last impinges, be examined by a delicate thermo- 
 scope, first when it is in a position to transmit the 
 polarised beam coming from the first plate, and 
 secondly when it has been turned round through 
 an arc of 90, and the polarised beam is absorbed. 
 I expected that, if the experiment were carefully 
 performed, the temperature of the second plate 
 would be more raised in the second case than in 
 the first, and that it might afford interesting 
 results when tried with light of different colours. 
 I met with difficulties in procuring a suitable 
 apparatus, and was endeavouring to overcome 
 them when I found that Knoblauch had, to some 
 extent, realised this result. He finds that, when a 
 solar beam, polarised in a certain plane, is trans- 
 mitted perpendicularly to the axis of a crystal 
 of brown quartz or tourmaline, the heat is trans- 
 mitted in a smaller proportion than when the beam
 
 LIGHT. 157 
 
 passes along the direction of the axis of the 
 crystal. 
 
 It is generally as far as I am aware, univer- 
 sally true that, while light continues as light, 
 even though reflected or transmitted by different 
 media, little or no heat is developed ; and, as far 
 as we can judge, it would appear that, if a medium 
 were perfectly transparent, or if a surface perfectly 
 reflected light, not the slightest heating effect would 
 take place ; but, wherever light is absorbed, then 
 heat takes its place, affording us apparently an 
 instance of the conversion of light into heat, and 
 of the fact that the force of light is not, in fact, 
 absorbed or annihilated, but merely changed in 
 character, becoming in this instance converted into 
 heat by impinging on solid matter, as in the in- 
 stance mentioned in treating of heat, this force was 
 shown to be converted into light by impinging on 
 solid matter. But the different effects of different 
 substances in transmitting and reflecting heat and 
 light greatly modify this view. One experiment, 
 indeed, of Melloni, already mentioned, would seern 
 to show that light may exist in a condition in 
 which it does not produce heat, which our instru- 
 ments are able to detect; but some doubt has 
 recently been thrown on the accuracy of this 
 experiment ; probably the substances themselves 
 through which the light is transmitted would be 
 found to have been heated.
 
 158 CORRELATION OF PHYSICAL FORCES. 
 
 The recipient body, or that upon which light 
 impinges, seems to exercise nearly as important an 
 influence on our perceptions of light as the emittent 
 body, or that from which the light first proceeds. 
 The recent experiments of Sir John Herschel and 
 Mr. Stokes show that radiant impulses, which, 
 falling on certain bodies, give no effect of light, 
 become luminous when falling on other bodies. 
 
 Thus, let ordinary solar light be refracted by a 
 prism (the best material for which is quartz), and 
 the spectrum received on a sheet of paper, or of 
 white porcelain ; looking on the paper, the eye 
 detects no light beyond the extreme violet rays. 
 If, therefore, an opaque body be interposed so as 
 just to cut off the whole visible spectrum, the paper 
 would be dark or invisible, with the exception of 
 some slight illumination from light reflected from 
 dust in the air and from surrounding bodies. Sub- 
 stitute for that portion of the paper which wan be- 
 yond the visible spectrum a piece of glass tinged 
 by the oxide of uranium, and the glass is perfectly 
 visible ; so with a bottle of sulphate of quinine, or 
 of the juice of horse-chestnuts, or even paper soaked 
 in these latter solutions. Other substances exhibit 
 in different degrees this effect termed fluorescence ; 
 and among the substances which have hitherto been 
 .considered perfectly analogous as to their appear- 
 ance when illuminated, notable differences are dis- 
 covered. Thus it appears that emanations which
 
 LIGHT. 159 
 
 give no impression of light to the eye, when im- 
 pinging on certain bodies, become luminous when 
 impinging on others. We might imagine a room so 
 constructed that such emanations alone are per- 
 mitted to enter it, which would be dark or light ac- 
 cording to the substance with which the walls were 
 coated, though in full daylight the respective coatings 
 of the walls would appear equally white ; or, without 
 altering the coating of the walls, the room exposed 
 to one class of rays might be rendered dark by win- 
 dows which would be transparent to another class. 
 If, instead of solar light, the electrical light be em- 
 ployed for similar experiments, an equally striking 
 effect can actually be produced. A design, drawn 011 
 white paper with a solution of sulphate of quinine 
 and tartaric acid, is invisible by ordinary light, 
 but appears with beautiful distinctness when illu- 
 minated by the electric light. Thus, in pronoun- 
 cing upon a luminous effect, regard must be had 
 to the recipient as well as to the emittent body. 
 That which is, or becomes, light when it falls 
 upon one body is not light when it falls upon 
 another. Probably the retinae of the eyes of dif- 
 ferent persons differ to some extent in a similar 
 manner; and the same substance, illuminated by 
 the same spectrum, may present different appear- 
 ances to different persons, the spectrum appearing 
 more elongated to the one than to the other, so 
 that what is light to the one is darkness to the
 
 160 CORRELATION OF PHYSICAL FORCES. 
 
 other. A dependence on the recipient body in;iy 
 also, to a great extent, be predicated of heat. Let 
 two vessels of water, the contents of the one clear 
 and transparent, of the other tinged by some colour- 
 ing matter, be suspended in a summer's sun ; in a 
 very short time a notable difference of temperature 
 will be observed, the coloured having become 
 much hotter than the clear liquid. If the first 
 vessel be placed at a considerable distance from 
 the surface of the earth, and the second near the 
 surface, the difference is still more considerable. 
 Carrying on this experiment, and suspending the 
 first over the top of a high mountain, and the 
 second in a valley, we may obtain so great a dif- 
 ference of temperature, that animals whose or<::in- 
 ization is suited for the one temperature could not 
 live in the other, and yet both are exposed to the 
 same luminous rays at the same time, and sub- 
 stantially at the same distance from the eminent 
 body the substance nearer the sun is in lint 
 colder than the more remote. So, with regard to 
 the medium transmitting the influence: a green- 
 house may have its temperature considerably varied 
 by changing the glass of which its roof is made. 
 
 These effects have an important bearing on ! 
 certain cosmical questions which have lately been 
 much discussed, and should induce the greatest 
 caution in forming opinions on such subjects as 
 light and heat on the sun's surface, the tempera- 

 
 LIGHT. 161 
 
 ture of the planets, &c. This may depend as 
 
 much upon their physical constitution as upon 
 
 their distance from the sun. Indeed, the planet 
 
 Mars gives us a highly probable argument for this ; 
 
 for, notwithstanding that it is half as far again 
 
 from the sun as the earth is, the increase of the 
 
 white tracts at its poles during its winter, and 
 
 their diminution during its summer, show that the 
 
 temperature of the surface of this planet oscillates 
 
 about that of the freezing point of water, as do 
 
 the analogous zones of our planet. It is true in 
 
 this we assume that the substance thus changing 
 
 its state is water, but considering the many close 
 
 I analogies of this planet with the earth, and the 
 
 ! identity in appearance of these very effects with 
 
 j what takes place on the earth, it seems not an 
 
 i improbable assumption. 
 
 So it by no means necessarily follows, that be- 
 | cause Venus is nearer to the sun than the earth, 
 that planet is hotter than our globe. The force 
 I emitted by the sun may take a different character 
 at the surface of each different planet, and require 
 different organisms or senses for its appreciation. 
 Myriads of organised beings may exist impercep- 
 tible to our vision, even if we were among them; 
 and we might be equally imperceptible to them ! 
 
 However vain it may be, in the present state of 
 science, to speculate upon such existences, it is 
 equally vain to assume identity or close approxi- 
 M
 
 162 CORRELATION OF PHYSICAL FORCES. 
 
 mations to our own forms in those beings which 
 may people other worlds. Reasoning from anal- 
 ogy or from final causation, if that be admitted, 
 we may feel convinced that the gorgeous globes 
 of the universe are not unpeopled deserts; but 
 whether the denizens of other worlds are more or 
 less powerful, more or less intelligent, whether 
 they have attributes of a higher or lower class 
 than ourselves,, is at present an utterly hopeless 
 guessing. 
 
 Specific gravity and intelligence have no neces- 
 sary connexion. On our own planet five senses, 
 and a mean density equal to that of water, are not 
 invariably associated with intellectual or moral 
 greatness, and the many arguments which have 
 been used to prove that suns and planets other 
 than the eartli are uninhabited, or not inhabited by 
 intellectual beings, might, mutatis mutandis, equally 
 be used by the denizens of a sun or planet to 
 prove that this world was uninhabited. 
 
 Men are too apt, because they are men, because 
 their existence is the one thing of all importance 
 to themselves, to frame schemes of the universe as 
 though it was formed for man alone : painted by 
 an artist of the sun, a man might not represent so 
 prominent an object of creation as he does when 
 represented by his own pencil. 
 
 Light was regarded, by what was termed the 
 corpuscular theory, as being in itself matter or a
 
 
 LIGHT. 163 
 
 specific fluid emanating from luminous bodies, and 
 producing the effects of sensation by impinging on 
 the retina. This theory gave way to the undula- 
 tory one, which is generally adopted in the present 
 day, and which regards light as resulting from the 
 undulation of a specific fluid to which the name of 
 ether has been given, which hypothetic fluid is 
 supposed to pervade the universe, and to permeate 
 the pores of all bodies. 
 
 In a Lecture delivered in January 1842, when 
 I first publicly advanced the views advocated in 
 this Essay, I stated that it appeared to me more 
 consistent with known facts to regard light as result- 
 ing from a vibration or motion of the molecules 
 of matter itself, rather than from a specific ether 
 pervading it; just .as sound is propagated by the 
 vibrations of wood, or as waves are by water. I am 
 not here speaking of the character of the vibrations 
 of light, sound, or water, which are doubtless very 
 different from each other, but am only comparing 
 them so far as they illustrate the propagation of 
 force by motion in the matter itself. 
 
 I was not aware, at the time that I first adopted 
 the above view, and brought it forward in my 
 Lectures, that the celebrated Leonard Euler had 
 published a somewhat similar theory ; and, though 
 I suggested it without knowing that it had been 
 previously advanced, I should have hesitated in 
 reproducing it had I not found that it was sanc- 
 
 M 2
 
 164 CORRELATION OF PHTSICAL FORCES. 
 
 tioned by so eminent a mathematician as Eulenj 
 who cannot be supposed to have overlooked any- 
 irresistible argument against it the more o in a 
 matter so much controverted and discussed as the 
 undulatory theory of light was in his time. 
 
 Although this theory has been considered de| 
 fcctive by a philosopher of high repute, I cannot 
 see the force of the arguments by which it hag 
 been assailed; and therefore, for the .present! 
 though with diffidence, I still adhere to it. The 
 fact itself of the correlation of the different raodeJ 
 of force is to my mind a very cogent argument in 
 favour of their being affections of the same matt- r; 
 and though electricity, magnetism, and heat might 
 be viewed as produced by undulations of the same 
 either as that by means of which light is supposed 
 to be produced, yet this hypothesis offers great, r 
 difficulties with regard to the other affections than 
 with regard to light : many of these difficulties I 
 have already alluded to when treating of electri- 
 city; thus conduction and non-conduction are not 
 explained by it ; the transmission of electricity 
 through long wires in preference to the air which 
 surrounds them, and which must be at least 
 equally pervaded by the ether, is irreconcilable 
 with such an hypothesis. The phenomena ex- 
 hibited by these forces afford, as I think, equally 
 strong evidence with those of light, of ordinary 
 matter acting from particle to particle, and having 
 no action at sensible distances. I have already
 
 LIGHT. 165 
 
 instanced the experiments of Faraday on electrical 
 induction, showing it to be an action of contigu- 
 ous particles, which are strongly in favour of this 
 view, and many experiments which I have made on 
 the voltaic arc, some of which I have mentioned in 
 this Essay, are, to my mind, confirmatory of it. 
 
 If it be admitted that one of the so-called 
 imponderables is a mode of motion, then the fact 
 of its being able to produce the others, and be 
 produced by them, renders it highly difficult to 
 conceive some as molecular motions and others as 
 fluids or undulations of an ether. To the main 
 objection of Dr. Young, that all bodies would 
 have the properties of solar phosphorus if light 
 consisted in the undulations of ordinary matter, it 
 may be answered that so many bodies have this 
 property, and with so great variety in its duration, 
 that non constat all may not have it, though for a 
 time so short that the eye cannot detect its dura- 
 tion. M. E. Becquerel has made many experi- 
 ments which support this view; the fact of the 
 phosphorescence by insolation of a large number 
 of bodies, and the phenomena of fluorescence 
 afford evidence of dense matter being thrown into 
 a state of undulation, or at all events molecularly 
 affected by the impact of light, and is there- 
 fore an argument in support of the view which 
 I am advocating. Dr. Young admits that the 
 phenomena of solar phosphorus appear to re- 
 semble greatly the sympathetic sounds of musical
 
 166 CORRELATION OF PHYSICAL FORCES. 
 
 instruments, which are agitated by other sounds 
 conveyed to them through the air, and I am not 
 aware that he gives any explanation of these effects 
 on the ethereal hypothesis. 
 
 Some curious experiments of M. Niepce de 
 Victor seem also to present an analogy in lumiiK 
 phenomena to sympathetic sounds. An engra^ 
 which has been kept for some days in the dark if 
 half covered by an opaque screen, and then exposed 
 to the sun ; it is then removed from the light, the 
 screen taken away, and the engraving placed op- 
 posite, and at a short distance from, photographic 
 paper: an in verted image of that portion of the 
 engraving which has been exposed to the sun is 
 produced on the photographic paper, while the part 
 which had been covered by the screen is not repro- 
 duced. If the engraving, after exposure, is allowed 
 to remain in contact with white paper for some 
 hours, and the white paper is then placed upon 
 photographic paper, a faint image of the exposed 
 portion of the engraving is reproduced; similar 
 results are produced by mottled marble exposed to 
 the sun; an invisible tracing on paper by a fluo- 
 rescent body, sulphate of quinine, is, after inso- 
 lation, reproduced on photographic paper, &c. In- 
 solated paper retains the power of producing an] 
 impression for a very long period, if it is kept in ail 
 opaque tube hermetically closed. 
 
 It is right to observe that these effects are
 
 
 LIGHT. 167 
 
 supposed by many to be due to chemical emanations 
 proceeding from the substances exposed to the sun, 
 and which are believed to have undergone some che- 
 mical change by this exposure ; this would still be a 
 molecular effect, but it is desirable to await further 
 experiment before forming a decided opinion. 
 
 The analogies in the progression of sound and 
 light are very numerous : each proceed in straight 
 lines, until interrupted; each is reflected in the 
 same manner, the angles of incidence and reflexion 
 being equal; each is alternately nullified and 
 doubled in intensity by interference ; each is ca- 
 pable of refraction when passing from media of 
 different density : this last effect of sound, long ago 
 theoretically determined, has been experimentally 
 proved by .Mr. Sondhauss, who constructed a lens 
 of films of collodion, which, when filled with 
 carbonic acid, enabled him to hear the ticking of a 
 watch placed at one focus of the lens, the ear of 
 ! the experimenter being at the opposite focus. The 
 ticking was not heard when the watch was moved 
 aside from the focal point, though it remained at 
 : an equal distance from the ear. An experiment of 
 ', M. Dove" seems, indeed, to show an effect of polari- 
 sation of sound. 
 
 The phenomena presented by heat, viewed ac- 
 cording to the dynamic theory, cannot be explained 
 by the motion of an imponderable ether, but 
 involve the molecular actions of ordinary ponder-
 
 168 CORRELATION OF PHYSICAL FORCES. 
 
 able matter. The doctrine of propagation by un- 
 dulations of ordinary matter is very generally ad- 
 mitted by those who support the dynamical theory 
 of heat ; but the analogies of the phenomena pre- 
 sented by heat and light are so close, that I cannot 
 see how a theory applied to the one agent should 
 not be applicable to the other. When heat is 
 transmitted, reflected, refracted, or polarised, can 
 we view that as an affection of ordinary matter, 
 and when the same effects take place with li^ht, 
 view the phenomena as produced by an imponder-j 
 able ether, and by that alone ? 
 
 An objection that immediately occurs to the : 
 mind in reference to the ethereal hypothesis of 
 light is, that the most porous bodies are opaque; 
 cork, charcoal, pumice stone, dried and moist 
 wood, &c., all very porous and very light, are all 
 opaque. This objection is not so superficial as it 
 might seem at first sight. The theory which as- 
 sumes that light is an undulation of an ethereal 
 medium pervading gross matter, assumes the dis- 
 tances between the molecules or atoms of matter 
 to be very great. Matter has been likened by 
 Democritus, and by many modern philosophers, to 
 the starry firmament, in which, though the indivi- 
 dual monads are at immense distances from each 
 other, yet they have in the aggregate a character I 
 of unity, and are firmly held by attraction in their 
 respective positions and at definite distances.
 
 LIGHT. 169 
 
 Now, if matter be built up of separate molecules, 
 then, as far as our knowledge extends, the lightest 
 bodies -would be those in which the molecules are 
 at the greatest distances, and those in which any 
 undulation of a pervading medium would be the 
 least interfered with by the separated particles 
 such bodies should consequently be the most trans- 
 parent. 
 
 If, again, the analogy of the starry firmament 
 held good, in this case an undulation or wave pro- 
 portioned to the individual monads would be 
 broken up by the number of them, and the very 
 appearance of continuity which results, as in the 
 milky way, from each point of vision being occupied 
 by one of the monads, would show that at some 
 portion of its progress the wave is interrupted by 
 one of them, so that the whole may be viewed in 
 some respect as a sheet of ordinary matter inter- 
 posed in the ethereal expanse. 
 
 Even then, if it be admitted that a highly elastic 
 medium pervades the interspaces, the separate 
 masses as a whole must exercise an important 
 influence on the progress of the wave. 
 
 Sound or vibrations of air meeting with a screen, 
 or, as it were, sponge of diffused particles, would be 
 broken up and dispersed by them ; but if they be 
 sufficiently continuous to take up the vibration 
 and propagate it themselves, the sound continues 
 comparatively unimpaired.
 
 170 CORRELATION OF PHYSICAL FORCES. 
 
 With regard, however, to liquid and gaseous 
 bodies, there are very great difficulties in viewing 
 them as consisting of separate and distant mole- 
 cules. If, for instance, we assume with Youn.tr 
 that the particles in water are comparatively 
 as distant from each other as 100 men would IK; 
 if dispersed at equal distances over the surface < >t' 
 England, the distance of these particles, when the 
 water is expanded into steam, would be increased 
 more than forty times, so that the 100 men woul( 
 be reduced to two, and by further increasing 
 temperature this distance may be indefinitely 
 increased; adding to the effects of temperatiu 
 rarefaction by the air-pump, we may again increase! 
 the distance, so that, if we assume any original 
 distance, we ought, by expansion, to increase it to 
 a point at which the distance between molecule and 
 molecule should become measurable. But no extent 
 of rarefaction, whether by heat or the air-pun i|>, 
 or both, makes the slightest change in the apparent 
 continuity of matter ; and gases, I find, retain their 
 peculiar character, as far as a judgment of it can 
 be formed from its effect on the electric spark- 
 throughout any extent of rarefaction which can 
 experimentally be applied to them : thus the electric 
 spark in protoxide of nitrogen, however attenuated, 
 presents a crimson tint, that in carbonic oxide 
 greenish tint. 
 
 Without, however, entering on the metaphysica
 
 LIGHT. 171 
 
 enquiry as to the constitution of matter (or 
 whether the atomic philosophers or the followers 
 of Boscovich are right), a problem which probably 
 human appliances will never solve ; and even 
 admitting that an ethereal medium, not absolutely 
 imponderable as asserted by many, but of extreme 
 tenuity, pervades matter, still ordinary or non- 
 ethereal matter itself must exercise a most im- 
 portant action upon the transmission of light ; and 
 Dr. Young, who opposed the theory of Euler, that 
 light was transmitted by undulations of gross 
 matter itself, just as sound is, was afterwards 
 obliged to call to his assistance the vibrations of 
 the ponderable matter of the refracting media, to 
 explain why rays of all colours were not equally 
 refracted, and other difficulties. One of his argu- 
 ments in support of the existence of a permeating 
 ether was, 'that a medium resembling in many 
 properties that which has been denominated Ether 
 does exist, is undeniably proved by the phenomena 
 of electricity.' This seems to me, if I may venture 
 to say so of anything proceeding from so eminent a 
 man, scarcely logical : it is supporting one hypothesis 
 by another, and considering that to be proved which 
 its most strenuous advocates admit to be sur- 
 rounded by very many difficulties. 
 
 If it be said that there is not sufficient elasticity 
 in ordinary matter for the transmission of undula- 
 tions with such velocity as light is known to travel,
 
 172 CORRELATION OF PHYSICAL FORCES. 
 
 this may be so if the vibrations be supposed exactly 
 analogous to those of sound ; but that molecular 
 motion can travel with equal and even greater 
 velocity than light, is shown by the rapidity with 
 which electricity traverses a metal wire where each 
 particle of metal is undoubtedly affected. It lias, 
 moreover, been shown by the experiments of Mr. 
 Latiiner Clarke upon a length of wire of 760 miles, 
 that whatever be the intensity of electrical currents, 
 they are propagated with the same velocity pro- 
 vided the effects of lateral induction be the same 
 a striking analogy with one of the effects observed 
 in the propagation of light and sound. The effects 
 observed by MM. Fizeau and Foucault, of the slower 
 progression of light in proportion as the transmit- 
 ting medium is more dense, seem to me in favour 
 of the view here advocated ; as a greater degree of 
 heat would be produced by light in proportion to 
 the density of the medium, force would be thus 
 carried off, and the molecular system disturbed so 
 that the progress of the motion should be more 
 slow; but so many considerations enter into this 
 question, and the phenomena are so extremely com- ; 
 plex, that it would be rash to hazard any positive 
 opinion. 
 
 Dr. Young ultimately came to the conclusion 
 that it was simplest to consider the ethereal 
 medium, together with the material atoms of the 
 substance, as constituting together a compound
 
 LIGHT. 173 
 
 medium denser than pure ether, but not more 
 elastic. Ether might thus be viewed as perform- 
 ing the functions which oil does with tracing 
 paper, giving continuity to the particles of gross 
 matter, and in the interplanetary spaces forming 
 itself the medium which transmits the undu- 
 lations. 
 
 Since the period when Huyghens, Euler, and 
 Young, the fathers of the undulatory theory, 
 applied their great minds to this subject, a mass of 
 experimental data has accumulated, all tending to 
 establish the propositions, that whenever matter 
 transmitting or reflecting light undergoes a 
 structural change, the light itself is affected, and 
 that there is a connection or parallelism between 
 the change in the matter and the change in the 
 aifection of light, and conversely that light will 
 modify or change the structure of matter and 
 impress its molecules with new characteristics. 
 
 Transparency, opacity, refraction, reflection, and 
 colour were phenomena known to the ancients, but 
 sufficient attention does not appear to have been 
 paid by them to the molecular states of the bodies 
 producing these effects ; thus the transparency or 
 opacity of a body appears to depend entirely 
 upon its molecular arrangement. If striae occur 
 in a lens or glass through which objects are 
 viewed, the objects are distorted: increase the 
 number of these striae, the distortion is so in-
 
 174 CORRELATION OF PHYSICAL FORCES. 
 
 creased that the objects become invisible, and tin; 
 glass ceases to be transparent, though remaining 
 translucent; but alter completely the molecular 
 structure, as by slow solidification, and it becomes 
 opaque. Take, again, an example of a liquid and a 
 gas : a solution of soap is transparent, air is trans- 
 parent, but agitate them together so as to form a 
 froth or lather, and this, though consisting of two 
 transparent bodies, is opaque ; and the reflection of 
 light from the surface of these bodies, when so inter- 
 mixed, is strikingly different from its reflection 
 before mixture, in the one case giving to the eye a 
 mere general effect of whiteness, in the other the 
 images of objects in their proper shapes and colours. 
 To take a more refined instance: nitrogen is 
 perfectly colourless, oxygen is perfectly colourless, 
 but chemically united in certain proportions they 
 form nitrous acid, a gas which has a deep orange- 
 brown colour. I know not how the colour of this 
 gas, or of such gases as chlorine or vapour of 
 iodine, can be accounted for by the ethereal hy- 
 pothesis, without calling in aid molecular affec- 
 tions of the matter of these erases. 
 
 O 
 
 Colour in many instances depends upon thj 
 thickness of the plate or film of transparent 
 matter upon which light is incident; as in all those 
 cases which are termed the colours of thin platesJ 
 of which the soap bubble affords a beautiful 
 instance.
 
 LIGHT. 175 
 
 When we arrive at the more recent discoveries 
 of double refraction and polarisation, the effects of 
 light are found to trace out as it were the struc- 
 ture of the matter affected, and the crystalline 
 form of a body can be determined by the effects 
 which a minute portion of it exercises on a ray of 
 light. 
 
 Let a piece of good glass be placed in what is called 
 apolariscope (i.e. an instrument in which light that 
 has undergone polarisation is transmitted through 
 the substance to be examined, and the emergent 
 light is afterwards submitted to another substance 
 capable of polarising light, or, as it is termed, an 
 analyser), no change in effect will be observed. 
 Remove the glass, heat it and suddenly or quickly 
 cool it so as to rend.er it unannealed, in which state 
 its molecules are in a state of tension or strain, and 
 the glass highly brittle, and on replacing it in the 
 polariscope, a beautiful series of colours is percep- 
 tible. Instead of subjecting the glass to heat and 
 sudden cooling, let it be bent or strained by me- 
 chanical pressure, and the colours will be equally 
 visible, modified according to the direction of the 
 flexure, and indicating by their course the curves 
 where the molecular state has been changed by 
 pressure. So if tough glue be elongated and 
 allowed to cool in a stretched state, it doubly 
 refracts light, and the colours are shown as in the 
 instance of glass.
 
 176 CORRELATION OF PHYSICAL FORCES. 
 
 Submit a series of crystals to the same exami- 
 nation, and different figures will be formed by 
 different crystals, bearing a constant and definite 
 relation to the structure of the particular crystal 
 examined, and to the direction in which, with 
 reference to crystalline form, the ray crosses the 
 crystal. 
 
 In the crystallised salts of paratartaric acid, : 
 M. Pasteur noticed two sets of crystals which 
 were hemihedral in opposite directions, i. e. the 
 crystals of one set were to those of the other as to 
 then* own image reflected in a mirror; on making 
 a separate solution of each of these classes of 
 crystals, he found that the solution of the one cla>s 
 rotated the plane of polarisation to the right, while 
 that of the other class rotated it to the left, and 
 that a mixture in proper proportions of the two 
 solutions produced no deviation in the plane of 
 polarisation. Yet all these three solutions are 
 what is termed isomeric, that is, have as far as 
 can be discovered the same chemical constitution. 1 
 
 In the above, and in innumerable other cases, itl 
 is seen that an alteration in the structure of a 
 transparent substance alters the character and 
 effects of the transmitted light. The phenomena 
 of photography prove that light alters the struc-j 
 ture of matter submitted to it ; with regard even! 
 to vision itself, the persistence of images on the 
 retina of the eye would seem to show that its* 

 
 
 LIGHT. 177 
 
 strucure is changed by the impact of light, the 
 luminous impressions being as it were branded oil 
 the retina, and the memory of the vision being the 
 scar of such brand. The science of photography 
 has reference mainly to solid substances, yet there 
 are many instances of liquid and gaseous bodies 
 being changed by the action of light : thus hydro- 
 cyanic acid, a liquid, undergoes a chemical change 
 and deposits a solid carbonaceous compound by 
 the action of light. Chlorine and hydrogen gases, 
 when mixed and preserved in darkness, do not 
 unite, but when exposed to light rapidly combine, 
 forming hydrochloric acid. 
 
 The above facts and many others might have 
 been given go far to connect light with motion 
 of ordinary matter, and to show that many of the 
 evidences which our senses receive of the existence 
 of light result from changes in matter itself. When 
 the matter is in the solid state, these changes are 
 more or less permanent; when in the liquid or 
 gaseous state, they are temporary in the greater 
 number of instances, unless there be some chemical 
 change effected, which is, as it were, seized upon 
 during its occurrence, and a resulting compound 
 formed, which is more stable than the original 
 compound or mixture. 
 
 I might weary my reader with examples, show- 
 ing that, in every case which we can trace out, the 
 effects of light are changed by any and every 
 
 N
 
 178 CORRELATION OF PHYSICAL FORCES. 
 
 change of structure, and that light has a definite 
 connection with the structure of the bodies affected 
 by it. I cannot but think that it is a strong 
 assumption to regard, ether, a purely hypothetical 
 creation, as changing its elasticity for each change 
 of structure, and to regard it as penetrating the 
 pores of bodies of whose porosity we have in many 
 cases no proof; the which pores must, moreovrr, 
 have a definite and peculiar communication, also 
 assumed for the purpose of the theory. 
 
 Ether is a most convenient medium for hypo- 
 thesis : thus, if to account for a given phenomenon 
 the hypothesis requires that the ether be more 
 elastic, it is said to be more elastic ; if more dense,? 
 it is said to be more dense ; if it be required by 
 hypothesis to be less elastic, it is pronounced to be 
 less elastic ; and so on. 
 
 The advocates of the ethereal hypothesis certainly 
 have this advantage, that the ether, being hypo- 
 thetical, can have its characters modified or changed 
 without any possibility of disproof either of its 
 existence or modifications. 
 
 It may be that the refined mathematical labours 
 on light, as on electricity, have given an undue 
 and adventitious strength to the hypotheses on 
 which they are based. 
 
 An objection to which the view I have been 
 advocating is open, and at first sight a formidable 
 one, is the necessity involved in it of an universal
 
 LIGHT. 179 
 
 plenum; for if light, heat, electricity, &c., be 
 affections of ordinary matter, then matter must 
 be supposed to be everywhere where these phe- 
 nomena are apparent, and consequently there can 
 be no vacuum. 
 
 These forces are transmitted through what are 
 called vacua, or through interplanetary spaces, 
 where matter, if it exist, must be in a highly 
 attenuated state. 
 
 It may be safely stated that hitherto all attempts 
 at procuring a perfect vacuum have failed. The 
 ordinary air-pump gives us only highly rarefied 
 air ; and, by the principle of construction, even of 
 the best, the operation depends upon the indefinite 
 expansion of the volume of air in the receiver; 
 even in the vacuum which is formed in this, so 
 great is the tendency of matter to fill up space, 
 that I have observed distilled water contained in a 
 vessel within the exhausted receiver of a good air- 
 puinp has a taste of tallow, derived from the grease, 
 or an essential oil contained in it, which is used to 
 form an air-tight junction between the edges of the 
 receiver and the pump-plate. 
 
 The Torricellian vacuum, or that of the ordinary 
 barometer, is filled with the vapour of mercury; 
 but it might be worth the trouble to ascertain 
 what would be the effect of a good Torricellian 
 : vacuum, when the mercury in the tube is frozen, 
 H -2
 
 180 CORRELATION OF PHYSICAL FORCES. 
 
 which might, without much difficulty, be now 
 effected by the use of solid carbonic acid and ether ; 
 the only probable difficulty would be the different 
 rates of contraction of mercury and glass, at such 
 a degree of cold, and more particularly the con- 
 traction of mercury at the period of its solidifi- 
 cation. Davy, indeed, endeavoured to form a 
 vacuum, in a somewhat similar manner, over fused 
 tin, with but partial success; he also made many 
 other attempts to obtain a perfect vacuum; his 
 mam object being to ascertain what would be the 
 effect of electricity across empty space : he admits 
 that he could not succeed in procuring a vacuum, 
 but found electricity much less readily conducted 
 or transmitted by the best vacuum he could pro- 
 cure than by the ordinary Boylean vacuum. 
 
 Morgan found no conduction by a good Torri- 
 cellian vacuum ; and, although Davy does not seem 
 to place much reliance on Morgan's experiments, 
 there was one point in which they were less liable 
 to error than those of Davy. Morgan, whose ex- 
 periments seem to have been carefully conducted, 
 operated with hermetically - sealed glass tubes 
 and by induced electricity, while Davy sealed a 
 platinum wire into the extremity of the tube in 
 which he sought to produce a vacuum. I have 
 found in very numerous experiments which I made 
 to exclude air from water, that platinum wires, 
 most carefully sealed into glass, allow liquids to
 
 LIGHT. 181 
 
 pass between them and the glass ; and this gives 
 some reason to believe that gases may equally 
 pass through; indeed, I have observed such effect 
 in the gas battery when it has been in action 
 for a long period. Davy supposed that the par- 
 ticles of bodies may be detached, and so produce 
 electrical effects in a vacuum; and such effects 
 would more readily take place in his experiments, 
 where a wire projected into the exhausted space, 
 than in Morgan's, where the induced electricity 
 was diffused over the surface of the glass. 
 
 M. Masson found that the barometric vacuum 
 does not conduct a current of electricity, or even a 
 discharge, unless the tension is considerable and 
 sufficient to detach particles from the electrodes; 
 and by adopting a plan of Dr. Andrews, viz. absorb- 
 ing carbonic acid by potash, Mr. Gassiot has 
 succeeded hi forming vacua across which the 
 powerful discharge from the Rhumkorf coil will 
 not pass. 
 
 The odour which many metals, such as iron, tin, 
 and zinc emit, and the so-called thermographic 
 radiations, can hardly be explained upon any other 
 theory than the evaporation of an infmitesimally 
 small portion of the metal itself. 
 
 So universal is the tendency of matter to diffuse 
 itself into space, that it gave rise to the old saying 
 that nature abhors a vacuum ; an aphorism which, 
 though cavilled at and ridiculed by the self-sum-
 
 182 CORRELATION OF PHYSICAL FORCES. 
 
 ciency of some modern philosophers, contains in a 
 terse though somewhat metaphorical form of ex- 
 pression, a comprehensive truth, and evinces a large 
 extent of observation in those who, with few of 
 the advantages which we possess, first generalised 
 by tins sentence the facts of which they had become 
 cognisant. 
 
 It has been argued that if matter were capable 
 of infinite divisibility, the earth's atmosphere would 
 have no limit, and that consequently portions of 
 it would exist at points of space where the attrac- 
 tion of the sun and planets would be greater than 
 that of the earth, and whence it would fly off to those 
 bodies and form atmospheres around them. This 
 was supposed to be negatived by the argument of 
 the well-known paper of Dr. Wollaston; in which, 
 from the absence of notable refraction near the 
 margin of the sun. and of the planet Jupiter, he con- 
 sidered himself entitled to conclude that the expan- 
 sion of the earth's atmosphere had a definite limit, 
 and was balanced at a certain point by gravitation : 
 this deduction has been shown to be inconclusive 
 by Dr. Whewell, and has also been impugned upon 
 other grounds by Dr. Wilson. There is a points! 
 not adverted to in these papers, and which Wol-1 
 laston does not seem to have considered, viz. that I 
 there is no evidence that the apparent discs of thcf 
 sun and of Jupiter show us their real discs on 
 bodies. Sir W. Herschel regards the margin of
 
 
 LIGHT. 183 
 
 the visible discs as that of clouds or a peculiar 
 state of atmosphere, and the rapidly changing 
 character of the apparent surfaces renders some 
 such conclusion necessary. If this be so, refraction 
 of an occulted star could not be detected at all 
 events, in the denser portion of the atmosphere. 
 
 Sir W. Herschel's observations go to prove that 
 the sun and "Jupiter have dense atmospheres, while 
 Wollaston's were believed to prove that they have 
 no appreciable atmospheres. 
 
 If it be admitted, or considered proved, that the 
 sun and planets have atmospheres and little doubt 
 now exists on this point then the grounds upon 
 which Wollaston founded his arguments are un- 
 tenable; and there appears no reason why the 
 atmosphere of the different planets should not be, 
 with reference to each other, in a state of equili- 
 brium. Ether, or the highly-attenuated matter 
 existing in the interplanetary spaces, being an ex- 
 pansion of some or all of these atmospheres, or of the 
 more volatile portions of them, would thus furnish 
 matter for the transmission of the modes of motion 
 which we call light, heat, &c. ; and possibly minute 
 portions of these atmospheres may, by gradual 
 changes, pass from planet to planet, forming a link 
 of material communication between the distant 
 monads of the universe. 
 
 The assumption of the universal presence of 
 matter is common to the theory of the transmis-
 
 184 CORKELATION OF PHYSICAL FORCES. 
 
 sion of light by the undulations of ordinary matter 
 and to the other two theories, which equally 
 suppose the non-existence of a vacuum; for, 
 according to the emissive or corpuscular theory, 
 the vacuum is filled by the matter itself, of light, 
 heat, &c. ; according to the ethereal it is filled 
 by the all-penetrating ether. Of the existence of 
 matter in the interplanetary spaces we have some 
 evidence in the diminishing periods of comets; 
 and where, from its highly attenuated state, the 
 character of the medium by which the forces are 
 conveyed cannot be tested, the term ether is a 
 most appropriate generic name for such medium. 
 
 Newton has some curious passages on the sub- 
 ject matter of light. In the ' Queries to the Optics' 
 he says : 
 
 * Are not gross bodies and light convertible into 
 one another, and may not bodies receive much of 
 their activity from the particles of light which 
 enter their composition? * * * The changing 
 of bodies into light and light into bodies is very 
 conformable to the course of nature, which seems 
 delighted with transmutations. Water, which is 
 a very fluid, tasteless salt, she changes by heat 
 into vapour, which is a sort of air, and by cold 
 into ice, which is a hard, pellucid, brittle, fusible 
 stone, and this stone returns into water by heat, 
 and vapour returns into water by cold. * * * 
 And, among such various and strange transmuta-
 
 
 LIGHT. 185 
 
 tions, why may not nature change bodies into 
 light, and light into bodies ?' 
 
 Newton has here seemingly in his mind the 
 emissive theory of light ; but the passages might 
 be applied to either theory ; the analogy he saw in 
 the change of state of matter, as in ice, water, and 
 vapour, with the hypothetic change into light, is 
 very striking, and would seem to show that he 
 regarded the change or transmutation of which he 
 speaks as one analogous to the known changes of 
 state, or consistence, in ordinary matter. 
 
 The difference between the view which I am 
 advocating and that of the ethereal theory as 
 generally enunciated is, that the matter which in 
 the interplanetary spaces serves as the means of 
 transmitting by its undulations light and heat, I 
 should regard as possessing the qualities of ordi- 
 nary, or as it has sometimes been called gross, 
 matter, and particularly weight ; though, from its 
 extreme rarefaction, it would manifest these pro- 
 perties in an indefinitely small degree; whilst, 
 near to or on the surface of the earth, that matter 
 attains a density cognisable by our means of 
 experiment, and the dense matter is itself, in 
 great part, the conveyer of the undulations in 
 which these agents consist. Doubtless, in very 
 many of the forms which matter assumes it is 
 porous, and pervaded by more volatile essences, 
 which may differ as much in kind as matter
 
 136 COEKELATION OF PHYSICAL FORCES. 
 
 does. In these cases a composite medium, such 
 as that indicated by Dr. Young, would result; 
 but even on such a supposition, the denser 
 matter would probably exercise the more impor- 
 tant influence on the undulations. Returning 
 to the somewhat strained hypothesis, that the 
 particles of dense matter in a so-called solid are 
 as distant as the stars in heaven, still a certain 
 depth or thickness of such solid would present 
 at every point of space a particle or rock in the 
 successive progress of a wave, which particles, to 
 carry on the movement, must vibrate in unison 
 with it. 
 
 At the utmost, our assumption, on the one hand, 
 is, that wherever light, heat, &c., exist, ordinary 
 matter exists, though it may be so attenuated that 
 we cannot recognise it by the tests of other forces, 
 such as gravitation ; and that to the expansibility of 
 matter no limit can be assigned. On the other 
 hand, a specific matter without weight must be 
 assumed, of the existence of which there is no 
 evidence, but in the phenomena for the explanation 
 of which its existence is supposed. To account 
 for the phenomena the ether is assumed, and to 
 prove the existence of the ether the phenomena 
 are cited. For these reasons, and others above 
 given, I think that the assumption of the univer- 
 sality of ordinary matter is the least gratuitous. 
 
 Clvliv TI rov Travroc KCfov Tre'Xet ouSe
 
 LIGHT. 187 
 
 A question has often occurred to me and possibly 
 to others : Is the continuance of a luminous impulse 
 in the interplanetary spaces perpetual, or does it 
 after a certain distance dissipate itself and become 
 lost as light I do not mean by mere divergence 
 directly as the squares of the distances it travels, 
 but does the physical impulse itself lose force as it 
 proceeds? Upon the view I have advocated, and 
 indeed upon any undulatory hypothesis, there must 
 be some resistance to its progress; and unless the 
 matter or ether in the interplanetary spaces be 
 infinitely elastic, and there be no lateral action of 
 a ray of light, there must be some loss. That it is 
 exceedingly minute is proved by the distance light 
 travels. Stars whose parallax is ascertained are at 
 such a distance from the earth that their light, travel- 
 ling at the rate of 192,500 miles in a second, takes 
 more than ten years to reach the earth ; so that we 
 see them as they existed ten years ago. The distance 
 of most visible stars is probably far greater than this, 
 and yet their brilliance is great, and increases when 
 their rays are collected by the telescope in propor- 
 tion ceteris paribus to the area of the object-glass 
 or speculum. There is, however, an argument of 
 a somewhat speculative character, by which light 
 would seem to be lost or transformed into some 
 other force in the interplanetary spaces. 
 
 Every increase of space -penetrating power in the 
 telescope gives us a new field of visible stars. If
 
 188 CORRELATION OF PHYSICAL FORCES. 
 
 tliis expansion of the stellar universe go on in- 
 definitely and no light be lost, then, assuming the 
 fixed stars to be of an average equal brightness 
 with our sun, and no light lost other than by 
 divergence, the night ought to be equally luminous 
 with the day ; for though the light from each point 
 diminishes in intensity as the square of the distance, 
 the number of luminous points would increase as 
 the square of the distance, and thus fill up the 
 whole space around us ; and if every point of space 
 is occupied by an equally brilliant point of light, 
 the distance of the points 'from us becomes im- 
 material. The loss of light intercepted by stellar 
 bodies would make no difference in the total 
 quantity of light, for each of these would yield 
 from its own self-luminosity at least as much 
 light as it intercepted. Light may, however, be in- 
 tercepted by non-luminous bodies, such as planets; 
 but, making every allowance for these, it is difficult 
 to understand why we get so little light at night 
 from the stellar universe, without assuming that 
 some light is lost in its progress through space 
 not lost absolutely, for that would be an anni- 
 hilation of force but converted into some other 
 mode of motion. 
 
 It may be objected that this hypothesis assumes 
 the stellar universe to be illimitable : if pushed to 
 its extreme so as to make the light of night equal 
 that of day provided no stellar light be lost, it does
 
 LIGHT. 189 
 
 make this assumption ; but even this is a far more 
 rational assumption to make than that the stellar 
 universe is limited. Our experience gives no indi- 
 cation of a limit ; each improvement in telescopic 
 power gives us new realms of stars or of nebula?, 
 which, if not stellar clusters, are at all events self- 
 luminous matter ; and if we assume a limit, what is 
 it? We cannot conceive a physical boundary, for 
 then immediately comes the question, what bounds 
 the boundary? and to suppose the stellar universe 
 to be bounded by infinite space or by infinite chaos, 
 that is to say, to suppose a spot for it would then 
 become so of matter in definite forms, with definite 
 forces, and probably teeming with definite organic 
 beings, plunged in a universe of nothing, is to my 
 mind at least far more unphilosophical than to 
 suppose a boundless universe of matter existing in 
 forms and actions more or less analogous to those 
 which, as far as our examination goes, pervade 
 space. But without speculating on topics in which 
 the mind loses itself, it may not unreasonably be 
 expected that a greater amount of light would 
 reach us from the surrounding self-luminous 
 spheres were not some portion lost as light, by 
 its action on the medium which conveys the 
 impulses. What force this becomes, or what it 
 effects, it would be vain to speculate upon.
 
 190 CORRELATION OF PHYSICAL FORCES. 
 
 MAGNETISM. 
 
 MAGNETISM, as was proved by the important dis- 
 covery of Faraday, will produce electricity, but 
 with this peculiarity that in itself it is static; 
 and, therefore, to produce a dynamic force, motion 
 must be superadded to it : it is, in fact, directive, 
 not motive, altering the direction of other forces, 
 but not, in strictness, initiating them. It is diffi- 
 cult to convey a definite notion of the force of 
 magnetism, and of the mode in wliich it affects 
 other forces. The following illustration may give 
 a rude idea of magnetic polarity. Suppose a 
 number of wind- vanes, say of the shape of arrows, 
 with the spindles on which they revolve arranged 
 in a row, but the vanes pointing in various direc- 
 tions: a wind blowing from the same point with 
 an uniform velocity will instantly arrange tlirsc 
 vanes in a definite direction, the arrow-heads or 
 narrow parts pointing one way, the swallow-tails 
 or broad parts another. If they be delicately sus- 
 pended on their spindles, a very gentle breeze will 
 so arrange them, and a very gentle breeze will 
 again deflect them ; or, if the wind cease, and they 
 have been originally subject to other forces, 8iich
 
 MAGNETISM. 191 
 
 as gravity from unequal suspension, they will 
 return to irregular positions, themselves creating 
 a slight breeze by their return. Such a state of 
 things will represent the state of the molecules of 
 soft iron; electricity acting on them not indeed 
 in straight lines, but in a definite direction pro- 
 duces a polar arrangement, which they will lose as 
 soon as the dynamic inducing force is removed. 
 
 Let us now suppose the vanes, instead of turn- 
 ing easily, to be more stiffly fixed to the axles, so 
 as to be turned with difficulty : it will require a 
 stronger wind to move them and arrange them 
 definitely ; but when so arranged, they will retain 
 their position; and should a gentle breeze spring 
 up in another direction, it will not alter their 
 position, but will, itself be definitely deflected. 
 Should the conditions of force and stability be 
 intermediate, both the breeze and the vanes will be 
 slightly deflected ; or, if there be no breeze, and the 
 spindles be all moved in any direction, preserving 
 their linear relation, they will themselves create a 
 breeze. Thus it is with the molecules of hard 
 iron or steel in permanent magnets; they are 
 polarised with greater difficulty than those of soft 
 iron, but, when so polarised, they cannot be 
 affected by a feeble current of electricity. Again, 
 if the magnets be moved, they themselves origi- 
 nate a current of electricity; and, lastly, the 
 magnetic polarity and the electric current may
 
 192 CORRELATION OF PHYSICAL FORCES. 
 
 be both mutually affected, if the degrees of mo- 
 bility and stability be intermediate. 
 
 The above instance will, of course, be taken 
 only as an approximation, and not as binding me 
 to any closer analogy than is generally expected 
 of a mechanical illustration. It is difficult to 
 convey by words a definite idea of the dual or 
 antithetic character of force involved in the term 
 polarity. The illustration I have employed may, 
 I hope, somewhat aid in elucidating the manner in 
 which magnetism acts on the other dynamic forces ; 
 i. e., definitely directing them, but not initiating 
 them, except while in motion. 
 
 Magnets being moved in the direction of lines, 
 joining their poles, produce electrical currents in 
 such neighbouring bodies as are conductors of 
 electricity, in directions transverse to the line <>f 
 motion; and if the direction of motion or the 
 position of the magnetic poles be reversed, th<> 
 current of electricity flows in a reverse direction. 
 So if the magnet be stationary, conducting bodies 
 moved across any of the lines of magnetic force 
 (i. e. lines in the direction of which the mutual 
 action of the poles of the magnet would place 
 minute portions of iron) have currents of electricity 
 developed in them, the direction of which is de- 
 pendent upon that of the motion of the substance 
 with reference to the magnetic poles. Thus, as 
 bodies affected by an electrical current are defi-
 
 MAGNETISM. 193 
 
 nitely moved by a magnet in proximity to them, 
 so conversely bodies moved near a magnet have an 
 electrical current developed in them. Magnetism 
 can, then, through the medium of electricity, pro- 
 duce heat, light, and chemical affinity. Motion it 
 can directly produce under the above conditions ; 
 i.e. a magnet being itself moved will move other 
 ferreous bodies : these will acquire a static con- 
 dition of equilibrium, and be again moved when 
 the magnet is also moved. By motion or arrested 
 motion only, could the phenomena of magnetism 
 ever have become known to us. A magnet, how- 
 ever powerful, might rest for ever unnoticed and 
 unknown, unless it were moved near to iron, or 
 iron moved near to it, so as to come within the 
 sphere of its attraction. 
 
 But even with other than either magnetic or 
 electrified substances, all bodies will be moved 
 when placed near the poles of very powerful 
 magnets some taking a position axially, or in 
 the line from pole to pole of the magnet; others 
 equatorially, or in a direction transverse to that 
 line the former being attracted, the latter 
 apparently repelled, by the poles of the magnet. 
 These effects, according to the views of Faraday, 
 show a generic difference between the two classes 
 of bodies, magnetics and diamagnetics ; according 
 to others, a difference of degree or a resultant of 
 magnetic action ; the less magnetic substance 
 o
 
 194 CORRELATION OF PHYSICAL FORCES. 
 
 being forced into a transverse position by the 
 magnetisation of the more magnetic medium which 
 surrounds it. 
 
 According to the view given above, magnetism 
 may be produced by the other forces, just as the 
 vanes in the instance given are definitely deflected, 
 but cannot produce them except when in motion : 
 motion, therefore, is to be regarded in this case as 
 the initiative force. Magnetism will, however, 
 directly affect the other forces light, heat, and 
 chemical affinity, and change their direction or 
 mode of action, or, at all events, will so affect 
 matter subjected to these forces, that their direc- 
 tion is changed. Since these lectures were de- 
 livered, Faraday has discovered a remarkable 
 effect of the magnetic force in occasioning the 
 deflection of a ray of polarised light. 
 
 If a ray of polarised light pass through water, 
 or through any transparent liquid or solid which 
 does not alter or turn aside the plane of polarisa- 
 tion, and the column, say of water, through which 
 it passes be subjected to the action of a powerful 
 magnet, the line of magnetic force, or that which 
 would unite the poles of the magnet, being in the 
 same direction as the ray of polarised light, 
 the water acquires, with reference to the light, 
 similar, though not quite identical, properties 
 to oil of turpentine the plane of polarisation 
 is rotated, and the direction of this rotation
 
 MAGNETISM. 195 
 
 is changed by changing the direction of the 
 magnetic force: thus, if we suppose a polarised 
 ray to pass first in its course the north pole of the 
 magnet, then between that and the south pole it 
 will be deflected, or curved to the right ; while if 
 it meet the south pole first in its course, it will, in 
 its journey between that and the north pole, be 
 turned to the left. If the substance through 
 which the ray is transmitted be of itself capable of 
 deflecting the plane of polarisation, as, for in- 
 stance, oil of turpentine, then the magnetic influ- 
 ence will increase or diminish this rotation, 
 according to its direction. A similar effect to 
 this is observed with polarised heat when the 
 medium through which it is transmitted is sub- 
 jected to magnetic influence. 
 
 Whether this effect of magnetism is rightly 
 termed an effect upon light and heat, or is a 
 molecular change of the matter transmitting the 
 light and heat, is a question the resolution of 
 which must be left to the future ; at present, the 
 answer to it would depend upon the theory we 
 adopt. If the view of light and heat which I have 
 stated be adopted, then we may fairly say that 
 magnetism, in these experiments, directly affects 
 the other forces ; for light and heat being, accord- 
 ing to that view, motions of ordinary matter, 
 magnetism, in affecting these movements, affects 
 the forces which occasion them. If, however, the 
 
 o2
 
 196 CORRELATION OF PHYSICAL FORCES. 
 
 other theories be adhered to, it would be more 
 consistent with the facts to view these results as 
 exhibiting an action upon the matter itself, and 
 the heat and light as secondarily affected. 
 
 When substances are undergoing chemical 
 changes, and a magnet is brought near them, the 
 direction or lines of action of the chemical force 
 will be changed. There are many old experiments 
 which probably depended on this effect, but which 
 were erroneously considered to prove that perma- 
 nent magnetism could produce or increase che- 
 mical action: these have been extended and ex- 
 plained by Mr. Hunt and Mr. Wartmann, and are 
 now better understood. 
 
 The above cases are applicable to the subject of 
 the present Essay, inasmuch as they show a rela- 
 tion to exist between magnetic and the other 
 forces, which relation is, in all probability, reci- 
 procal; but in these cases there is not a pro- 
 duction of light, heat, or chemical affinity, by 
 magnetism, but a change in their direction or 
 mode of action. 
 
 There is, however, that which may be viewed as 
 a dynamic condition of magnetism ; i.e. its condi- 
 tion at the commencement and the termination, or 
 during the increment or decrement of its develop- 
 ment. While iron or steel is being rendered 
 magnetic, and as it progresses from its non- 
 magnetic to its maximum magnetic state, or
 
 MAGNETISM. 197 
 
 recedes from its maximum to zero, it exhibits a 
 dynamic force; the molecules are, it may be in- 
 ferred, in motion. Similar effects can then be pro- 
 duced to those which are produced by a magnet 
 whilst in motion. 
 
 An experiment which I published in 1845 
 tends, I think, to illustrate this, and in some 
 degree to show the character of the motion im- 
 pressed upon the molecules of a magnetic metal 
 at the period of magnetisation. A tube filled 
 with the liquid in which magnetic oxide of iron 
 had been prepared, and terminated at each end by 
 plates of glass, is surrounded by a coil of coated 
 wire. To a spectator looking through this tube a 
 flash of light is perceptible whenever the coil is 
 electrised, and less, light is .transmitted when the 
 electrical current ceases, showing a symmetrical 
 arrangement of the minute particles of magnetic 
 oxide while under the magnetic influence. 
 
 In this experiment it should be borne in mind, 
 that the particles of oxide of iron are not shaped 
 by the hand of man, as would be the case with 
 iron filings, or similar minute portions of magnetic 
 matter, but being chemically precipitated, are of 
 the form given to them by nature. 
 
 While magnetism is in the state of change above 
 described, it will produce the other forces ; but it 
 may be said, while magnetism is thus progressive, 
 some other force is acting on it, and therefore it
 
 198 CORRELATION OF PHYSICAL FORCES. 
 
 does not initiate : this is true, but the same may 
 be said of all the other forces ; they have no 
 commencement that we can trace. We must ever 
 refer them back to some antecedent force equal in 
 amount to that produced, and therefore the word 
 initiation cannot in strictness apply, but must 
 only be taken as signifying the force selected as 
 the first : this is another reason why the idea of 
 abstract causation is inapplicable to physical pro- 
 duction. To this point I shall again advert. 
 
 Electricity may thus be produced directly by 
 magnetism, either when the magnet as a mass is 
 in motion, or when its magnetism is commencing, 
 increasing, decreasing, or ceasing; and heat may 
 similarly be directly produced by magnetism. I 
 have, since the first edition of this Essay was 
 published, communicated to the Royal Society a 
 paper by which I think I have satisfactorily 
 proved, that whenever any metal susceptible of 
 magnetism is magnetised or demagnetised, its 
 temperature is raised. This" was shown, first, by 
 subjecting a bar of iron, nickel, or cobalt to the 
 influence of a powerful electro-magnet, which was 
 rapidly magnetised and demagnetised in reverse 
 directions, the electro-magnet itself being kept 
 cool by cisterns of water, so that the magnetic 
 metal subjected to the influence of magnetism was 
 raised to a higher temperature than the electro- 
 magnet itself, and could not, therefore, have
 
 MAGNETISM. 199 
 
 acquired its increased temperature by conduction 
 or radiation of heat from the electro-magnet; 
 and secondly, by rotating a permanent steel 
 magnet with its poles opposite to a bar of iron, a 
 thermo-electric pile being placed opposite the 
 latter. 
 
 Dr. Maggi covered a plate of homogeneous soft 
 iron with a thin coating of wax mixed with oil, 
 a tube traversed the centre through which the 
 vapour of boiling water was passed. The plate was 
 made to rest on the poles of an electro-magnet, 
 with card interposed. When the iron is not mag- 
 netised, the melted wax assumes a circular form, 
 the tube occupying the centre, but when the 
 electro-magnet is put in action, the curve marking 
 the boundary of the melted substance changes its 
 form and becomes elongated in a direction trans- 
 verse to the line joining the poles, showing that the 
 conducting power of the iron for heat is changed 
 by magnetisation. 
 
 Thus we get heat produced by magnetism and 
 the conduction of heat altered by it in a direction 
 having a definite .relation to the direction of 
 the magnetism. Is it necessary to call in aid 
 ether or the substance ' caloric ' to explain these 
 results ? is it not more rational to regard the 
 calorific effects as changes in the molecular 
 arrangements of the matter subjected to mag- 
 netism?
 
 200 CORRELATION OF PHYSICAL FORCES. 
 
 There is some probability that magnetism, in j 
 the dynamic state, either when the magnet is in, 
 motion, or when the magnetic intensity is varying, j 
 will also directly produce chemical affinity and 1 
 light, though up to the present time, such has not ; 
 been proved to be the case ; the reciprocal effect, j 
 also, of the direct production of magnetism by i 
 light and heat has not yet been experimentally 
 established. 
 
 I have used, in contradistinction, the terms 
 dynamic and static to represent the different states 
 of magnetism. The applications I have made of 
 these terms may be open to some exception, but 
 I know of no other words which will so nearly 
 express my meaning. 
 
 The static condition of magnetism resembles 
 the static condition of other forces: such as the 
 state of tension existing in the beam and cord of a 
 balance, or in a charged Leyden phial. The old 
 definition of force was, that which caused change 
 in motion ; and yet even this definition presents a 
 difficulty : in a case of static equilibrium, such, for 
 instance, as that which obtains in the two arms of 
 a balance, we get the idea of force without any 
 palpable apparent motion : whether there be really 
 an absence of motion may be a doubtful question, 
 as such absence would involve in this case perfect 
 elasticity, and, in all other cases, a stability wh'u-li, 
 in a long course of time, nature generally nega-
 
 MAGNETISM. 201 
 
 tives, showing, as I believe, an inseparable con- 
 nection of motion with matter, and an impossibility 
 of a perfectly immobile or durable state. So with 
 magnetism ; I believe no magnet can exist in an 
 absolutely stable state, though the duration of its 
 stability will be proportionate to its original resist- 
 ance to assuming a polarised condition. This 
 however, must be taken merely as a matter of 
 opinion : we have, in support of it, the general fact 
 that magnets do deteriorate in the course of years ; 
 and we have the further general fact of the insta- 
 bility, or fluxional state, of all nature, when we 
 have an opportunity of fairly investigating it at 
 different and remote periods : in many cases, how- 
 ever, the action is so slow that the changes escape 
 human observation, and, until this can be brought 
 to bear over a proportionate period of time, the 
 proposition cannot be said to be experimentally or 
 inductively proved, but must be left to the mental 
 conviction of those who examine it by the light of 
 already acknowledged facts. 
 
 All cases of static force present the same dif- 
 ficulty: thus, two springs pressing against each 
 other would be said to be exercising force ; and yet 
 there is no resulting action, no heat, no light, &c. 
 
 So if gas be compressed by a piston, at the time 
 of compression heat is given off; but when this is 
 abstracted, although the pressure continues, no 
 further heat is eliminated. Thus, by an equilibrium
 
 202 CORRELATION OF PHYSICAL FORCES. 
 
 produced by opposing forces, motion is locked up, 
 or in abeyance, as it were, and may be again de- 
 veloped when the forces are relieved from the ten- 
 sion. But in the first instance, in producing the 
 state of tension, force has to be employed ; and as 
 we have said in treating of mechanical force, so with 
 the other forces the original change which disturbs 
 equilibrium produces other changes which go on 
 without end. Thus, by the act of charging a 
 Leyden phial, the cylinder, the rubber, and the 
 adjoining portions of the electrical machine have 
 each and all their states changed, and thence pro- 
 duce changes in surrounding bodies ad infinitum 
 when the jar is discharged, converse changes are 
 again produced. 
 
 As with heat, light, and electricity, the daily 
 accumulating observations tend to show that each 
 change in the phenomena to which these names 
 are given is accompanied by a change either tem- 
 porary or permanent in the matter affected by 
 them; so many experiments on magnetism have 
 connected magnetic phenomena with a molecular 
 change in the subject matter. Thus M. Wertheim 
 has shown that the elasticity of iron and steel is 
 altered by magnetisation ; the co-efficient of elasti- 
 city in iron being temporarily, in steel permanently 
 diminished. 
 
 He has also examined the effects of torsion upon 
 magnetised iron, and concludes, from his experi-
 
 MAGNETISM. 203 
 
 ments, that in a bar of iron arrived at a state of 
 magnetic equilibrium, temporary torsion diminishes 
 the magnetism, and that the untwisting or return 
 to its primitive state restores the original degree 
 of magnetisation. 
 
 M. Guillemin observed that a bar slightly curved 
 by its own weight is straightened by being mag- 
 netised. Mr. Page and Mr. Marrion discovered 
 that a sound is emitted when iron or steel is 
 rapidly magnetised or demagnetised ; and Mr. Joule 
 found that a bar of iron is slightly elongated by 
 magnetisation . 
 
 Again, with regard to diamagnetic bodies. M. 
 Matteucci found that the mechanical compression 
 of glass altered the rotatory power of magnetism 
 upon a ray of polarised light which the glass trans- 
 mitted. He further considered that a change took 
 place in the temper of portions of glass which he 
 submitted to the influence of powerful magnets. 
 
 The same arguments which have been submitted 
 to the reader as to the other affections of matter 
 being modes of molecular motion, are therefore 
 equally applicable to magnetism.
 
 204 CORRELATION OF PHYSICAL FORCES. 
 
 CHEMICAL AFFINITY. 
 
 CHEMICAL AFFINITY, or the force by which dis- 
 similar bodies tend to unite and form compounds 
 differing generally in character from their con- 
 stituents, is that mode of force of which the human 
 mind has hitherto formed the least definite idea. 
 The word itself affinity is ill chosen, its mean- 
 ing, in this instance, bearing no analogy to its 
 ordinary sense ; and the mode of its action is con- 
 veyed by certain conventional expressions, no 
 dynamic theory of it worthy of attention having 
 been adopted. Its action so modifies and alters 
 the character of matter, that the changes it induces 
 have acquired, not perhaps very logically, a generic 
 contradistinction from other material changes, and 
 we thus use, as contradistinguished, the terms 
 physical and chemical. 
 
 The main distinction between chemical affinity 
 and physical attraction or aggregation, is the dif- 
 ference of character of the chemical compound from 
 its components. This is, however, but a vague line 
 of demarcation; in many cases, which would be 
 classed by all as chemical actions, the change of
 
 CHEMICAL AFFINITY. 205 
 
 character is but slight ; in others, as in the effects 
 of neutralisation, the difference of character would 
 be a result which would equally follow from 
 physical attraction of dissimilar substances, the 
 previous characters of the constituents depending 
 upon this very attraction or affinity : thus an acid 
 corrodes because it tends to unite with another 
 body ; when united, its corrosive power, i. e. its 
 tendency to unite, being satiated, it cannot, so to 
 speak, be further attracted, and it necessarily loses 
 its corrosive power. But there are other cases 
 where no such result could a priori be anticipated, 
 as where the attraction or combining tendency of 
 the compound is higher than that of any of its 
 constituents ; thus, who could, by physical reason- 
 ing, anticipate a substance like nitric acid from the 
 combination of nitrogen and oxygen ? 
 
 The nearest approach, perhaps, that we can form 
 to a comprehension of chemical action, is by re- 
 garding it (vaguely perhaps) as a molecular attrac- 
 tion or motion. It will directly produce motion of 
 definite masses, by the resultant of the molecular 
 changes it induces: thus, the projectile effects of 
 gunpowder may be cited as familiar instances of 
 motion produced by chemical action. It may be a 
 question whether, in this case, the force which 
 occasions the motion of the mass is a conversion of 
 the force of chemical affinity, or whether it is not, 
 rather, a liberation of other forces existing in a state 

 
 206 CORRELATION OF PHYSICAL FORCES. 
 
 of static equilibrium, and having been brought into 
 such state by previous chemical actions ; but, at all 
 events, through the medium of electricity chemical 
 affinity may be directly and quantitatively con- 
 verted into the other modes of force. By chemical 
 affinity, then, we can directly produce electricity ; 
 this latter force was, indeed, said by Davy to be 
 chemical affinity acting on masses: it appears, 
 rather to be chemical affinity acting in a definite 
 direction through a chain of particles ; but by no 
 definition can the exact relation of chemical affinity 
 and electricity be expressed ; for the latter, however 
 closely related to the former, yet exists where the 
 former does not, as in a metallic wire, which, when 
 electrified, or conducting electricity, is, neverthe- 
 less, not chemically altered, or, at least, not known 
 to be chemically altered. 
 
 Volta, the antitype of Prometheus, first enabled 
 us definitely to relate the forces of chemistry and 
 electricity. When two dissimilar metals in contact 
 are immersed in a liquid belonging to a certain 
 class, and capable of acting chemically on one of 
 them, what is termed a voltaic circuit is formed, 
 and, by the chemical action, that peculiar mode of 
 force called an electric current is generated, which 
 circulates from metal to metal, across the liquid, 
 and through the points of contact. 
 
 Let us take, as an instance of the conversion of 
 chemical force into electrical, the following, which
 
 CHEMICAL AFFINITY. 2U7 
 
 I made known some years ago. If gold be im- 
 mersed in hydrochloric acid, no chemical action 
 takes place. If gold be immersed in nitric acid, 
 no chemical action takes place ; but mix the two 
 acids, and the immersed gold is chemically attacked 
 and dissolved : this is an ordinary chemical action, 
 the result of a double chemical affinity. In hydro- 
 chloric acid, which is composed of chlorine and 
 hydrogen, the affinity of chlorine for gold being 
 less than its affinity for hydrogen, no change takes 
 place; but when the nitric acid is added, this 
 latter containing a great quantity of oxygen in a 
 state of feeble combination, the affinity of oxygen 
 for hydrogen opposes that of hydrogen for chlorine, 
 and then the affinity of the latter for gold is 
 enabled to act, the gold combines with the chlorine 
 and chloride of gold remains in solution in the 
 liquid. Now, in order to exhibit this chemical 
 force in the form of electrical force, instead of 
 mixing the liquids, place them in separate vessels 
 or compartments, but so that they may be in 
 contact, which may be effected by having a porous 
 material, such as unglazed porcelain, amianthus, &c., 
 between them. Immerse in each of these liquids a 
 strip or wire of gold : as long as these pieces of 
 gold remain separated, no chemical or electrical 
 eifect takes place ; but the instant they are brought 
 into metallic contact, either immediately or by 
 connecting each with the same metallic wire,
 
 208 CORRELATION OF PHYSICAL FORCES. 
 
 chemical action takes place the gold in the 
 hydrochloric acid is dissolved, electrical action 
 also takes place, the nitric acid is deoxidised by 
 the transferred hydrogen, and a current of elec- 
 tricity may be detected in the metals, or con- 
 necting metal, by the application of a galvano- 
 meter or any instrument appropriate for detecting 
 such effect. 
 
 There are few, if any, chemical actions which 
 cannot be experimentally made to produce elec- 
 tricity: the oxidation of metals, the burning of 
 combustibles, the combination of oxygen and 
 hydrogen, &c., may all be made sources of elec- 
 tricity. The common mode in which the electri- 
 city of the voltaic battery is generated is by the 
 chemical action of water upon zinc ; this action is 
 increased by adding certain acids to the water, 
 which enable it to act more powerfully upon the 
 zinc, or in some cases act themselves upon it; and 
 one of the most powerful chemical actions known 
 that of nitric acid upon oxidable metals is 
 that which produces the most powerful voltaic 
 battery, a combination which I made known in 
 the year 1839: indeed, we may safely say, that 
 when the chemical force is utilised, or not wasted, 
 but all converted into electrical force, the more 
 powerful the chemical action, the more powerful is 
 the electrical action which results. 
 
 Tf, instead of employing manufactured products
 
 CHEMICAL AFFINITY. 209 
 
 or eclucts, such as zinc and acids, we could realise 
 as electricity the whole of the chemical force 
 which is active in the combustion of cheap and 
 abundant raw materials, such as coal, wood, fat, 
 &c., with air or water, we should obtain one of 
 the greatest practical desiderata, and have at our 
 command a mechanical power in every respect 
 superior in its applicability to the steam engine. 
 
 I have shown that the flame of the common 
 blowpipe gives rise to a very marked electrical 
 current, capable not only of affecting the galvano- 
 meter, but of producing chemical decomposition: 
 two plates or coils of platinum are placed, the 
 one in the portion of the flame near the orifice 
 of the jet, or at the points where combustion 
 commences, the other in the full yellow flame 
 where combustion is at its maximum; this latter 
 should be kept cool, to enable a thermo-electric 
 current, which is produced by the different tem- 
 perature of the platinum plates, to co-operate 
 with the flame current; wires attached to the 
 plates of platinum form the terminals or poles. 
 By a row of jets a flame battery may be formed, 
 yielding increased effects; but in these experi- 
 ments, though theoretically interesting, so small a 
 fraction of the power, actually at work in the com- 
 bustion, has been thrown into an electrical form, 
 that there is no immediate promise of a practical 
 result.
 
 210 CORRELATION OF PHYSICAL FORCES. 
 
 The quantity of the electrical current, as 
 measured by the quantity of matter it acts upon 
 in its different phenomenal effects, is proportionate 
 to the quantity of chemical action which generated 
 it; and its intensity, or power of overcoming 
 resistance, is also proportionate to the intensity 
 of chemical affinity when a single voltaic pair is 
 employed, or to the number of reduplications 
 when the well-known instrument called the voltaic 
 battery is used. 
 
 The mode in which the voltaic current is in- 
 creased in intensity by these reduplications, is in 
 itself a striking instance of the mutual relations 
 and dynamic analogies of different forces. Let a 
 plate of zinc or other metal possessing a strong 
 affinity for oxygen, and another of platinum or 
 other metal possessing little or no affinity for 
 oxygen, be partially immersed in a vessel, A, con- 
 taining dilute nitric acid, but not in contact with 
 each other; let platinum wires touching each of 
 these plates have their extremities immersed in 
 another vessel, B, containing also dilute nitric 
 acid : as the acid hi vessel A is decomposed, by the 
 chemical affinity of the zinc for the oxygen of the 
 acid, the acid in vessel B is also decomposed, 
 oxygen appearing at the extremity of the wire 
 which is connected with the platinum : the chemi- 
 cal power is conveyed or transferred through the 
 wires, and, abstracting certain local effects, for every
 
 CHEMICAL AFFINITY. 211 
 
 unit of oxygen which combines with the zinc in 
 the one vessel, a unit of oxygen is evolved from the 
 platinum wire in the other. The platinum wire is 
 thus thrown into a condition analogous to zinc, or 
 has a power given to it of determining the oxygen 
 of the liquid to its surface, though it cannot, as is 
 the case with zinc, combine with it under similar 
 circumstances. If we now substitute for the pla- 
 tinum wire, which was connected with the platinum 
 plate, a zinc wire, we have, in addition to the 
 determining tendency by which the platinum was 
 aifected, the chemical affinity of the oxygen in 
 vessel B for the zinc wire : thus we have, added to 
 the force which was originally produced by the 
 zinc of the combination in vessel A, a second force 
 produced by the zinc in vessel B, co-operating 
 with the first ; two pairs of zinc and platinum thus 
 connected produce, therefore, a more intense effect 
 than one pair; and if we go on adding to these 
 alternations of zinc, platinum, and liquid, we 
 obtain an indefinite exaltation of chemical power, 
 just as in mechanics we obtain accelerated motion 
 by adding fresh impulses to motion already gene- 
 rated. 
 
 The same rule of proportion which holds good 
 in chemical combinations also obtains in electrical 
 effects, when these are produced by chemical actions. 
 Dalton and others proved that the constituents of a 
 vast number of compound substances always bore 
 
 p2
 
 212 CORRELATION OF PHYSICAL FORCES. 
 
 a definite quantitative relation to each other : thus, 
 water, which consists of one part by weight of 
 hydrogen united to eight parts of oxygen, cannot 
 be formed by the same elements in any other than 
 these proportions; you can neither add to nor 
 subtract from the normal ratio of the elements, 
 without entirely altering the nature of the com- 
 pound. Further, if any element be selected as 
 unity, the combining ratios of other elements will 
 bear an invariable quantitative relation to that and 
 to each other : thus, if hydrogen be chosen as 1, 
 oxygen will be 8, chlorine will be 36 ; that is, 
 oxygen will unite with hydrogen in the propor- 
 tion of 8 parts by weight to 1, while chlorine 
 will unite with hydrogen in the proportion of 
 36 to 1, or with oxygen in the proportion of 
 36 to 8. Numbers expressing their combining 
 weights, which are thus relative, not absolute, 
 may, by a conventional assent as to the point of 
 unity, be fixed for all chemical reagents; and, 
 when so fixed, it will be found that bodies, at least 
 in inorganic compounds, generally unite in those 
 proportions, or in simple multiples of them : these 
 proportions are termed Equivalents. 
 
 Now a voltaic battery, which consists usually of 
 alternations of two metals, and a liquid capable of 
 acting chemically upon one of them, has, as we 
 have seen, the power of producing chemical action 
 in a liquid connected with it by metals upon which
 
 CHEMICAL AFFINITY. 213 
 
 this liquid is incapable of acting : in such case the 
 constituents of the liquid will be eliminated at the 
 surfaces of the immersed metals, and at a distance 
 one from the other. For example, if the two 
 platinum terminals of a voltaic battery be im- 
 mersed in water, oxygen will be evolved at one 
 and hydrogen at the other terminal, exactly in the 
 proportions in which they form water; while, to 
 the most minute examination, no action is per- 
 ceptible in the intervening stratum of liquid. It 
 was known before Faraday's tune that, while this 
 chemical action was going on in the subjected 
 liquid, a chemical action was going on in the cells 
 of the voltaic battery; but it was scarcely if at all 
 known that the amount of chemical action in the 
 one bore a constant relation to the amount of 
 action in the other. Faraday proved that it bore 
 a direct equivalent relation : that is, supposing the 
 battery to be formed of zinc, platinum, and water, 
 the amount of oxygen wliich united with the zinc 
 in each cell of the battery was exactly equal to the 
 amount evolved at the one platinum terminal, 
 while the hydrogen evolved from each platinum 
 plate of the battery was equal to the hydrogen 
 evolved from the other platinum terminal. 
 
 Supposing the battery to be charged with hydro- 
 chloric acid, instead of water, while the terminals 
 are separated by water, then for every 36 parts 
 by weight of chlorine which united with each plate
 
 214 CORRELATION OF PHYSICAL FORCES. 
 
 of zinc, eight parts of oxygen would be evolved 
 from one of the platinum terminals : that is, the 
 weights would be precisely in the same relation 
 which Dalton proved to exist in their chemical com- 
 bining weights. This relation applies to all liquids 
 capable of being decomposed by the voltaic electrical 
 force, thence called Electrolytes : and as no voltaic 
 effect is produced by liquids incapable of being 
 thus decomposed, it follows that voltaic action is 
 chemical action taking place at a distance, or 
 transferred through a chain of media, and that the 
 chemical equivalent numbers are the exponents of 
 the amount of voltaic action for corresponding 
 chemical substances. 
 
 As heat, light, magnetism, or motion, can be 
 produced by the requisite application of the electric 
 current, and as this is definitely produced by 
 chemical action, we get these forces very defi- 
 nitely, though not immediately, produced by chem- 
 ical action. Let us, however, here enquire, as we 
 have already done with respect to the other forces, 
 how far other forces may directly emanate from 
 chemical affinity. 
 
 Heat is an immediate product of chemical affi- 
 nity. I know of no exception to the general 
 proposition that all bodies in chemically combining 
 produce heat; i. e. if solution be not considered as 
 chemical action, and even in that case, when cold 
 results, it is from a change of consistence, as from
 
 CHEMICAL AFFINITY. 215 
 
 the solid to the liquid state, and not from chemical 
 action, 
 
 We shall find that the same view of the ex- 
 penditure of force which we have considered in 
 treating of latent heat holds good as to the ex- 
 penditure of chemical force when regarded with 
 reference to the amount of heat or repulsive force 
 which it engenders, the chemical force being here 
 exhausted by mechanical expansion that is, by 
 heat. Thus, in the chemical action of the ordi- 
 nary combustion of coal and oxygen, the expen- 
 diture of fuel will be in proportion to the ex- 
 pansibility of the substances heated ; water passing 
 freely into the state of steam will consume more 
 fuel than if it be confined and kept at a tem- 
 perature above its boiling point. 
 
 Why chemical action produces heat, or what 
 is the action of the molecules of matter when 
 chemically uniting, is a question upon which many 
 theories have been proposed and which may pos- 
 sibly be never more than approximately resolved. 
 
 Some authors explain it by the condensation 
 which takes place; but this will not account for 
 the many instances where, from the liberation of 
 gases, a great increase of volume ensues upon 
 chemical combustion, as in the familiar instance of 
 the explosion of gunpowder: others explain it as 
 resulting from the union of atmospheres of posi- 
 tive and negative electricity which are assumed to
 
 216 COKRELATION OF PHYSICAL FORCES. 
 
 jsurround the atoms of bodies; but this involves 
 hypothesis upon hypothesis. Dr. Wood has lately 
 thrown out the view of the heat of chemical action 
 which is more in accordance with a dynamic 
 theory of heat, and as such demands some notice. 
 Starting with his proposition, which I have pre- 
 viously mentioned, 'that the nearer the particles 
 of bodies are to each other the less they require to 
 move to produce a given motion in the particles 
 of another body,' his argument assumes something 
 of this form. 
 
 In the mechanical approximation of the particles 
 of a homogeneous body heat results ; the particles 
 a a of the body A would, by their approximation, 
 produce expansion in the neighbouring body B, 
 the more so in proportion as they themselves were 
 previously nearer to each other. In chemically 
 combining, a a the particles of A are brought into 
 very close proximity with b b the particles of B ; 
 heat should therefore result, and the greater 
 because the proximity may fairly be assumed to 
 be greater in the case of chemical combination 
 than in that of mechanical compression. In cases, 
 then, where there is no absolute diminution of 
 bulk ensuing on chemical combination, if the 
 greater proximity of the combining particles be 
 such that the correlative expansion ought to be 
 greater (if there were no chemical combination) 
 than that occupied by the total volume of the new
 
 CHEMICAL AFFINITY. 217 
 
 compound, an extra expanding power is evolved, 
 and heat or expansion ought to be produced in 
 surrounding bodies. In other words, if a a could 
 be brought by physical attraction as near each 
 other as they are by chemical attraction brought 
 near to b b, they would, from their increased 
 proximity, produce an expansive power ultra the 
 volume occupied by the actual chemical compound 
 A and B. The question, however, immediately 
 occurs, why should the volume of the compound 
 be limited and not occupy the full space equivalent 
 to the expanding power induced by the contraction 
 or approximation of the particles. As the distance 
 of the particles is the resultant of the contending 
 contracting and expanding powers, this result 
 ought to express, itself in terms of the actual 
 volume produced by the combination, which it 
 certainly does not. 
 
 Though I see some difficulties in Dr. Wood's 
 theory, and perhaps have not rightly conceived it, 
 his views have to my mind great interest, his 
 mode of regarding natural phenomena being ana- 
 logous to that which I have in this Essay, and 
 for many years, advocated, viz. to divest physical 
 science as much as possible of hypothetic fluids, 
 ethers, latent entities, occult qualities, &c. My 
 own notion of the heat produced by chemical 
 combination, though I scarcely dare venture an 
 opinion upon a subject so controverted, is, that it
 
 218 CORRELATION OF PHYSICAL FORCES. 
 
 is analogous to the heat of friction, that the 
 particles of matter in close approximation and 
 rapid motion inter se evolve heat as a continua- 
 tion of the motion interrupted by the friction or 
 intestinal motion of the particles: heat would 
 thus be produced, whether the resulting compound 
 were of greater or less bulk than the sum of the 
 components, though of course when the compound 
 is of greater bulk less heat would be apparent hi 
 neighbouring bodies, the expansion taking place in 
 one of the substances themselves I say in one 
 of them, for it is stated in books of authority that 
 there is no instance of two or more solids or 
 liquids, or a solid and a liquid, combining and 
 producing a compound which is entirely gaseous 
 at ordinary temperatures and pressures. The 
 substance gun-cotton, however, discovered by Dr. 
 Schoenbein, very nearly realises this proposition. 
 
 Dr. Andrews has arrived at the conclusion, after 
 careful experiment, that in chemical combinations 
 where acids and alkalies or analogous substances 
 are employed, the amount of heat produced is deter- 
 mined by the basic ingredient, and his experiments 
 have received general assent; although it should 
 be stated that M. Hess arrived at contrary results, 
 the acid constituent according to his experiments 
 furnishing the measure of the heat developed. 
 
 Light is directly produced by chemical action, 
 as in the flash of gunpowder, the burning of phos-
 
 CHEMICAL AFFINITY. 219 
 
 phorus in oxygen gas, and all rapid combustions : 
 indeed, wherever intense heat is developed, light 
 accompanies it. In many cases of slow combustion, 
 such as the phenomena of phosphorescence, the 
 light is apparently much more intense than the 
 heat; the former being obvious, the latter so diffi- 
 cult of detection that for a long time it was a 
 question whether any heat was eliminated ; and I 
 am not aware that, at the present day, any thermic 
 effects from certain modes of phosphorescence, such 
 as those of phosphorescent wood, putrescent fish, 
 &c., have been detected. 
 
 Chemical action produces magnetism whenever 
 it is thrown into a definite direction, as in the 
 phenomenon of electrolysis. I may adduce the 
 gas voltaic battery, as presenting a simple instance 
 of the direct production of magnetism by chemical 
 synthesis. Oxygen and hydrogen in that combi- 
 nation chemically unite ; but instead of combining 
 by intimate molecular admixture, as in the ordi- 
 nary cases, they act upon water, i. e. combined 
 oxygen and hydrogen, placed between them so as 
 to produce a line of chemical action ; and a magnet 
 adjacent to this line of action is deflected, and 
 places itself at right angles to it. What a chain 
 of molecules does here, there can be no doubt, all 
 the molecules entering into combination would 
 produce in ordinary chemical actions ; but in such 
 cases, the direction of the lines of combination
 
 220 CORRELATION OF PHYSICAL FORCES. 
 
 being irregular and confused, there is no general 
 resultant by which the magnet can be affected. 
 
 What the exact nature of the transference of 
 chemical power across an electrolyte is, we at 
 present know not, nor can we form any more defi- 
 nite idea of it than that given by the theory of 
 Grotthus. We have no knowledge as to the exact 
 nature of any mode of chemical action, and, for 
 the present, must leave it as an obscure action of 
 force, of which future researches may simplify our 
 apprehension. 
 
 We have seen that an equivalent or propor- 
 tionate electrical effect is produced by a given 
 amount of chemical action ; if we, in turn, produce 
 heat, magnetism and motion by the electricity 
 resulting from chemical action, we shall be able 
 to measure these forces far more accurately than 
 when they are directly produced, and thus to deduce 
 their equivalent relation to the initial chemical 
 action. Thus M. Favre, after ascertaining the 
 quantity of heat produced by the oxidation of a 
 given quantity of zinc, and finding, as have others, 
 that the heat is the same when evolved from a 
 voltaic battery by the same consumption of zinc 
 forming its positive element, makes the following 
 experiment. 
 
 A voltaic battery and electro-magnet are im- 
 mersed in calorimeters, and the heat produced when 
 the connection with the magnet is effected is noted.
 
 CHEMICAL AFFINITY. 221 
 
 The electro-magnet is then made to raise a weight, 
 and thus perform mechanical work ; and the heat 
 produced is again noted. It is found in the latter 
 case that less heat is evolved than in the former, 
 a certain quantity of heat has therefore been re- 
 placed by the mechanical work ; and by estimating 
 the amount of heat subtracted, and the amount of 
 work produced, he deduces the relative equivalent 
 of work to heat. These experiments give a pro- 
 duction of mechanical work by chemical action, 
 not, it is true, a direct production, but, as the heat 
 and work are in inverse ratios, and each has its 
 source in chemical action, they prove that they are 
 definite for a definite amount of chemical action, 
 and as each is produced respectively by electricity 
 and magnetism, these forces must also bear a 
 definite relation to the initial chemical force. 
 
 The doctrine of definite combining proportions, 
 which so beautifully serves to relate chemistry to 
 voltaic electricity, led to the atomic theory, which, 
 though adopted in its universality by a large 
 majority of chemists, presents great difficulties 
 when extended to all chemical combinations. 
 
 The equivalent ratios in which a great number 
 of substances chemically combine, hold good in so 
 many instances, that the atomic doctrine is believed 
 by many to be universally applicable, and called a 
 law; and yet, when followed in the combinations 
 .of substances whose mutual chemical attractions
 
 222 CORRELATION OF PHYSICAL FORCES. 
 
 are very feeble, the relation fades away, and is 
 sought to be recovered by applying a separate and 
 arbitrary multiplier to the different constituents. 
 
 Thus, when it was found that a vast number 
 of substances combined in definite volumes and 
 weights, and in definite volumes and weights only, 
 it was argued that their ultimate molecules or 
 atoms had a definite size, as otherwise there was 
 no apparent reason why this equivalent ratio 
 should hold good? why, for instance, water should 
 only be formed of two volumes or one unit by 
 weight of hydrogen, and of one volume or eight 
 units by weight of oxygen? why, unless there 
 were some ultimate limits to the divisibility of its 
 molecules, should not water, or a fluid substance 
 approximating to water in character, be formed by 
 a half, a third, or a tenth part of hydrogen, with 
 eight parts of oxygen? 
 
 It was perfectly consistent with the atomic view 
 that a substance might be formed with one part 
 combined with eight parts, or with sixteen, or 
 with twenty-four, for in such a substance there 
 would be no subdivision of the (supposed indi- 
 visible) molecule ; and this held good with many 
 compounds : thus fourteen parts by weight, say 
 grams of nitrogen, will combine respectively with 
 eight, sixteen, twenty-four, thirty-two, and forty 
 parts by weight, or grain?, of oxygen. 
 
 So, again, twenty-seven grains of iron will com-
 
 CHEMICAL AFFINITY. 223 
 
 bine with eight grains of oxygen or with twenty- 
 four grains, i.e. three proportionals of oxygen. 
 No compound is known in which twenty-seven 
 grains of iron will combine with two proportionals 
 or sixteen grains of oxygen; but this does not 
 much affect the theory, as such a compound may 
 be yet discovered, or there may be reasons at 
 present unknown why it cannot be formed. 
 
 But now comes a difficulty : twenty-seven parts 
 by weight of iron will combine with twelve parts 
 by weight of oxygen, and twenty-seven parts of 
 iron will also combine with ten and two-third 
 parts of oxygen. Thus, if we retain the unit of 
 iron we must subdivide the unit of oxygen, or if 
 we retain the unit of oxygen we must subdivide 
 the unit of iron, or we must subdivide both by a 
 different divisor. What then becomes of the notion 
 of an atom or molecule physically indivisible ? 
 
 If iron were the only substance to which this 
 difficulty applied, it might be viewed as an unex- 
 plained exception, or as a mixture of two oxides; 
 or recourse might be had to a more minute subdivi- 
 sion to form the units or equivalents of other sub- 
 stances; but numerous other substances fall under 
 a similar category; and in organic combinations, 
 to preserve the atomic nomenclature, we must apply 
 a separate multiplier or divisor to far the greater 
 number of the elementary constituents, i.e. we must 
 divide that which is, ex hypothesi, indivisible.
 
 224 CORRELATION OF PHYSICAL FORCES. 
 
 Thus, to take a more complex substance than 
 any formed by the combination of iron and oxy- 
 gen, let us select the substance albumen, composed 
 of carbon, hydrogen, nitrogen, oxygen, phosphorus, 
 and sulphur. In tins case we must either divide 
 the atoms of phosphorus and sulphur so as to 
 reduce them to small fractions, or multiply the atoms 
 of the other substances by extravagant numbers ; 
 thus to preserve the unit of one of the constituents 
 of this substance, chemists say it is composed of 
 400 atoms of carbon, 310 of hydrogen, 120 of 
 oxygen, 50 of nitrogen, 2 of sulphur, and 1 of 
 phosphorus. This is a somewhat extreme case, 
 but similar difficulties will be found in different 
 degrees to prevail among organic compounds ; in 
 very many no constituent can be taken as a unit to 
 which simple multiples of any of the others will 
 give their relative proportions. By the mode of 
 notation adopted, if any conceivable substance be 
 selected, it could, whatever be the proportions of 
 its constituents, be termed atomic. A solution of 
 an ounce of sugar in a pound of water, in a pound 
 and a half, in a pound and a quarter, in a pound 
 and a tenth, might be expressed in an atomic form, 
 if we select arbitrarily a multiplier or divisor. 
 
 It is true that in the case of solution, different 
 proportions can be united up to the point of satu- 
 ration without any difference in the character of 
 the compound, though the same may be predicated
 
 CHEMICAL AFFINITY. 225 
 
 to some extent of an acid and an alkali ; but even 
 where the steps are sudden, and compounds only 
 exist with definite proportions, they cannot, in a 
 multitude of cases, be reconciled with the true idea 
 of an atomic combination, i.e. one to one, one to 
 two, one to three, &c. 
 
 Although, therefore, nature presents us with 
 facts which show that there is some restrictive 
 law of combination which in numerous cases limits 
 the ratios in which substances will combine, nay, 
 further, shows many instances of a proportion 
 between the combining weights of one compound 
 and those of another; although she shows also a 
 remarkable simplicity in the combining volumes of 
 numerous gases, she also gives numerous cases to 
 which the doctrine of atomic combinations cannot 
 fairly be applied. 
 
 That there must be something in the constitu- 
 tion of matter, or in the forces which act on it, 
 to account for the per saltum manner in which che- 
 mical combinations take place, is inevitable ; but 
 the idea of atoms does not seem satisfactorily to 
 account for it. 
 
 By selecting a separate multiplier or divisor, 
 chemists may denote every combination in terms 
 derived from the atomic theory ; but they have 
 passed from the original law, which contemplated 
 only definite multiples, and the very hypothetic 
 expressions of atoms, which the apparently simple 
 Q
 
 226 CORRELATION OF PHYSICAL FORCES. 
 
 relations of combining weights first led them to 
 adopt, they are obliged to vary and to contradict 
 in terms, by dividing that which their hypothesis 
 and the expression of it assumed to be indivisible. 
 
 While, therefore, I fully recognise a great natu- 
 ral truth in the definite ratios presented by a vast 
 number of chemical combinations, and in the per 
 saltum steps in wliich nearly all take place, I 
 cannot accept as an argument in favour of an 
 atomic theory, those combinations which are made 
 to support it by the application of an arbitrary 
 notation. 
 
 A similar straining of theory seems gradually 
 obtaining in regard to the doctrine of compound 
 radicals. The discovery of cyanogen by Gay- 
 Lussac was probably the first inducement to the 
 doctrine of compound radicals: a doctrine which 
 is now generally, perhaps too generally, received 
 in organic chemistry. As, in the case of cyanogen, 
 a body obviously compound discharged in almost 
 all its reactions the functions of an element, so 
 in many other cases it was found that compound 
 bodies, in which a number of elements existed, 
 might be regarded as binary combinations, by^ 
 considering certain groups of these elements as a 
 compound radical ; that is, as a simple body when 
 treated of in relation to the other complex sub- 
 stances of which it forms part, and only as non- 
 elementary when referred to its internal consti- 
 tution.
 
 CHEMICAL AFFINITY. 227 
 
 Undoubtedly, by approximating in theory the 
 reactions of inorganic and of organic chemistry, 
 by keeping the mind within the limits of a beaten 
 path, instead of allowing it to wander through a 
 maze of isolated facts, the doctrine of compound 
 radicals has been of sendee ; but, on the other 
 hand, the indefinite variety of changes which may 
 be rung upon the composition of an organic sub- 
 stance, by different associations of its primary 
 elements, makes the binary constituents vary as 
 the minds of the authors who treat of them, and 
 makes their grouping depend entirely upon the 
 strength of the analogies presented to each indi- 
 vidual mind. From this cause, and from the ex- 
 treme license which has been taken in theoretic 
 groupings deduced from this doctrine, a serious 
 question arises whether it may not ultimately, un- 
 less carefully restricted, produce confusion rather 
 than simplicity, and be to the student an embar- 
 rassment rather than an assistance.
 
 228 CORRELATION OF PHYSICAL FORCES. 
 
 OTHER MODES OF FORCE. 
 
 CATALYSIS, or the chemical action induced by the 
 mere presence of a foreign body, embraces a class 
 of facts which must considerably modify many of 
 our notions of chemical action: thus oxygen and 
 hydrogen, when mixed in a gaseous state, will re- 
 main unaltered for an indefinite period; but the 
 introduction to them of a slip of clean platinum 
 will cause more or less rapid combination, without 
 being in itself in any respect altered. On the 
 other hand, oxygenated water, which is a com- 
 pound of one equivalent of hydrogen plus two of 
 oxygen, will, when under a certain temperature, 
 remain perfectly stable ; but touch it with plati- 
 num in a state of minute division, and it is in- 
 stantly decomposed, one equivalent of oxygen 
 being set free. Here, again, the platinum is un- 
 altered, and thus we have synthesis and analysis 
 effected apparently by the mere contact of a foreign 
 body. It is not improbable that the increased 
 electrolytic power of water by the addition of 
 some acids, such as the sulphuric and phosphoric, 
 where the acids themselves are not decomposed, 
 depends upon a catalytic effect of these acids;
 
 OTHER MODES OF FORCE. 229 
 
 but we know too little of the nature and rationale 
 of catalysis to express any confident opinion on 
 its modes of action, and possibly we may com- 
 prehend very different molecular actions under 
 one and the same name. In no case does catalysis 
 yield us new power or force: it only determines 
 or facilitates the action of chemical force, and, 
 therefore, is no creation of force by contact. 
 
 The force so developed by catalysis may be con- 
 verted into a voltaic form thus : in a single pair of 
 the gas battery previously alluded to, one portion 
 of a strip of platinum is immersed in a tube of oxy- 
 gen, the other in one of hydrogen, both the gases 
 and the extremities of the platinum being con- 
 nected by water or other electrolyte; a voltaic 
 combination is thus formed, and electricity, heat, 
 light, magnetism, and motion, produced at the will 
 of the experimenter. 
 
 In this combination we have a striking instance 
 of correlative expansions and contractions, analo- 
 gous, though in a much more refined form, to the 
 expansions and contractions by heat and cold de- 
 tailed in the early part of this essay, and illustrated 
 by the alternate actions of two bladders partially 
 filled with air : thus, as by the effect of chemical 
 combination in each pair of tubes of the gas battery 
 the gases oxygen and hydrogen lose their gaseous 
 character and shrink into water, so at the platinum 
 terminals of the battery, when immersed in water,
 
 230 CORBELATION OF PHYSICAL FORCES. 
 
 water is decomposed, and expands into oxygen and 
 hydrogen gases. The correlate of the force which 
 changes gas into liquid at one point of SJ>;UT 
 changes liquid into gas at another, and the exact 
 volume which disappears in the one place reappears 
 in the other; so that it would appear to an inex- 
 perienced eye as though the gases passed through 
 solid wires. 
 
 Gravitation, inertia, and aggregation, were but 
 cursorily alluded to in niy original lectures; their 
 relation to the other modes of force seemed to be 
 less definitely traceable ; but the phenomenal effects 
 of gravitation and inertia, being motion and resist- 
 ance to motion, in considering motion I have in 
 some degree included their relations to the other 
 forces. 
 
 To my mind gravitation would only produce 
 other force when the motion caused by it is dimi- 
 nished or ceases. Thus, if we suppose a meteor to 
 be a mass rotating in an orbit round the earth, 
 and with no resisting medium, then, as long as 
 that rotation continues, the motion of the meteoric 
 mass itself would be the exponent of the force 
 impelling it; if there be a resisting medium, part 
 of this motion would be arrested and taken up 
 by the medium, either as motion, heat, electricity, 
 or some other mode of force ; if the meteor ap- 
 proach the earth sufficiently to fall upon it, the 
 perceptible motion of the meteor is stopped, but
 
 OTHEE MODES OF FORCE. 231 
 
 is taken up by the earth which vibrates through 
 its mass ; part also reappears as heat in both earth 
 and meteor, and part in the change in the earth's 
 position consequent on its increase of gravity, and 
 so on. Gravitation is but the subjective idea, and 
 its relation to other modes of force seems to me to 
 be identical with that of pressure or motion. Thus, 
 when arrested motion produces heat, it matters 
 not whether the motion has been produced by a 
 falling body, i. e. by gravitation, or a body projected 
 by an explosive compound, &c. ; the heat will be the 
 same, provided the mass and velocity at the time 
 of arrest be the same. In no other sense can I 
 conceive a relation between gravitation and the 
 other forces, and, with all diffidence, I cannot agree 
 with those who consider there is a different sort of 
 link. 
 
 Mosotti has mathematically treated of the iden- 
 tity of gravitation with cohesive attraction, and 
 Pliicker has recently succeeded in showing that 
 crystalline bodies are definitely affected by mag- 
 netism, and take a position in relation to the lines 
 of magnetic force dependent upon their optical 
 axis or axis of symmetry. 
 
 What is termed the optic axis is a fixed direction 
 through crystals, in which they do not doubly re- 
 fract light, and which direction, in those crystals 
 which have one axis of figure, or a line around 
 which the figure is symmetrical, is parallel to the
 
 232 CORRELATION OF PHYSICAL FORCES. 
 
 axis of symmetry. When submitted to magnetic 
 influence such crystals take up a position, so that 
 their optic axis points diamagnetically or trans- 
 versely to the lines of magnetic force ; and when, 
 as is the case in some crystals, there is more than 
 one optic axis, the resultant of these axes points 
 diamagnetically. The mineral cyanite is influenced 
 by magnetism in so marked a manner that when 
 suspended it will arrange itself definitely with re- 
 ference to the direction of terrestrial magnetism, 
 and may, according to Pliicker, be used as a com- 
 pass-needle. 
 
 There is scarcely any doubt that the force wliich 
 is concerned in aggregation is the same wliich 
 gives to matter its crystalline form ; indeed, a vast 
 number of inorganic bodies, if not all, which appear 
 amorphous, are, when closely examined, found to be 
 crystalline in then* structure : we thus get a reci- 
 procity of action between the force which unites 
 the molecules of matter and the magnetic force, 
 and through the medium of the latter the correla- 
 tion of the attraction of aggregation with the other 
 modes of force may be established. 
 
 I believe that the same principles and mode of 
 reasoning as have been adopted in this essay might 
 be applied to the organic as well as the inorganic 
 world ; and that muscular force, animal and vege- 
 table heat, &c., might, and at some tune will, be 
 shown to have similar definite correlations ; but I
 
 OTHER MODES OF FORCE. 233 
 
 have purposely avoided this subject, as pertaining 
 to a department of science to which I have not 
 devoted my attention. I ought, however, while 
 alluding to this subject, shortly to mention some 
 experiments of Professor Matteucci, communicated 
 to the Royal Society in the year 1850, by which 
 it appears that whatever mode of force it be which 
 is propagated along the nervous filaments, this 
 mode of force is definitely affected by currents of 
 electricity. His experiments show that when a 
 current of positive electricity traverses a portion of 
 the muscle of a living animal in the same direction 
 as that in which the nerves ramify i.e. a direc- 
 tion from the brain to the extremities a muscular 
 contraction is produced in the limb experimented 
 on, showing that the nerve of motion is affected; 
 while, if the current, as it is termed, be made to 
 traverse the muscle in the reverse direction, or to- 
 wards the nervous centres, the animal utters cries, 
 and exhibits all the indications of suffering pain, 
 scarcely any muscular movement being produced ; 
 showing that in this case the nerves of sensation 
 are affected by the electric current, and therefore 
 that some definite polar condition exists, or is in- 
 duced, in the nerves, to which electricity is corre- 
 lated, and that probably this polar condition con- 
 stitutes or conveys nervous agency. There are 
 other analogies given in the papers of M. Matteucci, 
 and derived from the action of the electrical organs
 
 234 CORRELATION OF PHYSICAL FORCES. 
 
 of fishes, which tend to corroborate and develope 
 the same view. 
 
 By an application of the doctrine of the Cor- 
 relation of Forces, Dr. Caq^enter has shown how 
 a difficulty arising from the ordinary notions of 
 the development of an organised being from its 
 germ-cell may be lessened. It has been thought 
 by many physiologists that the nisus formativus, or 
 organising force of an animal or vegetable struc- 
 ture, lies dormant in the primordial germ-cell. ' So 
 that the organising force required to build up an 
 oak or a palm, an elephant or a whale, is concen- 
 trated in a minute particle only discernible by 
 microscopic aid.' 
 
 Certain other views of nearly equal difficulty 
 have been propounded. Dr. Carpenter suggests 
 the probability of extraneous forces, as heat, light, 
 and chemical affinity, continuously operating upon 
 the material germ ; so that all that is required in 
 this is a structure capable of receiving, directing, 
 and converting these forces into those which tend 
 to the assimilation of extraneous matter and the 
 definite development of the particular structure. 
 In proof of this position he shows how dependent 
 the process of genii development is upon tin- 
 presence and agency of external forces, particularly 
 heat and light, and how it is regulated by the f 
 measure of these forces supplied to it. 
 
 It certainly is far less difficult so to conceive the
 
 OTHER MODES OF FORCE. 235 
 
 supply of force yielded to organised beings in their 
 gradual process of growth, thah to suppose a store 
 of dormant or latent force pent up in a microscopic 
 monad. 
 
 As by the artificial structure of a voltaic bat- 
 tery, chemical actions may be made to co-operate 
 in a definite direction, so, by the organism of a 
 vegetable or animal, the mode of motion which 
 constitutes heat, light, &c., may, without extra- 
 vagance, be conceived to be appropriated and 
 changed into the forces which induce the absorp- 
 tion, and assimilation of nutriment, and into nervous 
 agency and muscular power. Indications of simi- 
 lar thoughts may be detected in the writings of 
 Liebig. 
 
 Some difficulty 'in studying the correlations of 
 vital with inorganic physical forces arises from 
 the effects of sensation and consciousness, pre- 
 senting a similar confusion to that alluded to, 
 when, in treating of heat, I ventured to suggest, 
 that observers are too apt to confound the sen- 
 sations with the phenomena. Let us apply some 
 of the considerations on force, given in the intro- 
 ductory portion of this essay, to cases where vitality 
 or consciousness intervenes. When a weight is 
 raised by the hand, there should, according to the 
 doctrine of non-creation of force, have been some- 
 where an expenditure equivalent to the amount of 
 gravitation overcome in raising the weight. That
 
 236 CORRELATION OF PHYSICAL FORCES. 
 
 there is expenditure we can prove, though in the 
 present state of science we cannot measure it. 
 Thus, prolong the effort, raise weights for an hour 
 or two, the vital powers sink, food, i. e. fresh 
 chemical force, is required to supply the exhaus- 
 tion. If tliis supply is withheld and the exertion 
 is continued, we see the consumption of force in 
 the supervening weakness and emaciation of the 
 body. 
 
 The consciousness of effort, which has formed a 
 topic of argument by some writers when treating 
 of force, and is by them believed to be that which 
 has originated the idea of force, may by the 
 physical student be regarded as feeling is in the 
 phenomena of heat and cold, viz. a sensation of 
 the struggle of opposing molecular motions in 
 overcoming the resistance of the masses to be 
 moved. When we say we feel hot, we feel cold, 
 we feel that we are exerting ourselves, our expres- 
 sions are intelligible to beings who are capable 
 of experiencing similar sensations ; but the physical 
 changes accompanying these sensations are not 
 thereby explained. Without pretending to know 
 what probably we shall never know, the actual 
 modus agendi of the brain, nerves, muscles, &c., 
 we may study vital as we do inorganic pheno- 
 mena, both by observation and experiment. Thus, 
 Sir Benjamin Brodie has examined the effect of 
 respiration on animal heat by inducing artificial
 
 OTHER MODES OF FORCE. 237 
 
 respiration after the spinal cord has been severed ; 
 in which case he finds the animal heat declines, 
 notwithstanding the continuance of the chemical 
 action of respiration, carbonic acid being formed as 
 usual; but he also finds that under such circum- 
 stances the struggles or muscular actions of the 
 animal are very great, and suificient probably to 
 account for the force eliminated by the chemical 
 action in digestion and respiration ; and Liebig, by 
 measuring the amount of chemical action in di- 
 gestion and respiration, and comparing it with the 
 labour performed, has to some extent established 
 their equivalent relations. 
 
 Mr. Helmholtz has found that the chemical 
 changes which take place in muscles are greater 
 when these are made to undergo contractions than 
 when they are in repose; and that, as would be 
 expected, the consumption of the matter of the 
 muscle, or, in other terms, the waste of excremen- 
 titious matter thrown off, is greater in the former 
 than in the latter case. 
 
 M. Matteucci has ascertained that the muscles 
 of recently killed frogs absorb oxygen and exhale 
 carbonic acid, and that when they are thrown into 
 a state of contraction, and still more when they 
 perform mechanical work, the absorption is in- 
 creased; and he even calculates the equivalents of 
 work so performed. 
 
 M. Beclard finds that the quantity of heat
 
 238 CORRELATION OF PHYSICAL FORCES. 
 
 produced by voluntary muscular contraction in 
 man is greater when that contraction is what he 
 terms static, that is, when it produces no external 
 work, but is effort alone, than when that effort and 
 contraction are employed dynamically, so as to 
 raise a weight or produce mechanical work. 
 
 Thus, though we may see no present promise of 
 being able to resolve sensations into their ultimate 
 elements, or to trace, physically, the link which 
 unites volition with exertion or effort, we may 
 hope to approximate the solution of these deeply 
 interesting questions. 
 
 In the same individual the chemical and physical 
 state of the secretions in the warm may be com- 
 pared with those in the cold parts of the body. 
 The changes hi digestion and respiration, when the 
 body is in a state of rest, may be compared with 
 those which obtain when it is in a state of activity. 
 The relations with external matter, maintaining 
 by the constant play of natural forces, the vital 
 nucleus, or the organisation by means of which 
 matter and force receive, for a definite period, 
 a definite incorporation and direction, may be as- 
 certained, while the more minute structural changes 
 are revealed to us by the ever improving powers of 
 the microscope ; and thus step by step we may learn 
 that which it is given to us to learn, boundless in 
 its range and infinite in its progress, and therefore 
 never giving a response to the ultimate Why?
 
 OTHER MODES OF FORCE. 239 
 
 As the first glimpse of a new star is caught 
 by the eye of the astronomer while directing his 
 vision to a different point of space, and disappears 
 when steadfastly gazed at, only to have its position 
 and figure ultimately ascertained by the employ- 
 ment of more penetrative powers, so the first scin- 
 tillations of new natural phenomena frequently 
 present themselves to the eye of the observer, 
 dimly seen when viewed askance, and disappear- 
 ing if directly looked for. When new powers of 
 thought and experiment have developed and cor- 
 rected the first notions, and given a character 
 to the new image, probably very different from the 
 first impression, fresh objects are again glanced 
 at in the margin of the new field of vision, 
 which in their turn have to be verified, and again 
 lead to new extensions; thus the effort to estab- 
 lish one observation leads to the imperfect per- 
 ception of new and wider fields of research ; and, 
 instead of approaching finality, the more we dis- 
 cover the more infinite appears the range of the 
 undiscovered !
 
 240 CORRELATION OF PHYSICAL FORCES. 
 
 CONCLUDING REMARKS. 
 
 I HAVE now gone through the affections of matter 
 to which distinct names have been given in our 
 received nomenclature: that other forces may be 
 detected, differing as much from them as they 
 differ from each other, is highly probable, and that 
 when discovered, and their modes of action fully 
 traced out, they will be found to be related inter 
 se, and to these forces as these are to each other, 
 I believe to be as far certain as certainty can be 
 predicted of any future event. 
 
 It may in many cases be u difficult question to 
 determine what constitutes a distinct affection of 
 matter or mode of force. It is highly probable 
 that different lines of demarcation would have been 
 drawn between the forces already known, had they 
 been discovered in a different manner, or first 
 observed at different points of the chain which 
 connects them. Thus, radiant heat and light are 
 mainly distinguished by the manner in which they 
 affect our senses: were they viewed according to 
 the way in which they affect inorganic matter, 
 very different notions would possibly be enter-
 
 CONCLUDING REMARKS. 241 
 
 tained of their character and relation. Electricity, 
 again, was named from the substance in which, 
 and magnetism from the district where, it first 
 happened to be observed, and a chain of inter- 
 mediate phenomena have so connected electricity 
 with galvanism that they are now regarded as 
 the same force, differing only in the degree of its 
 intensity and quantity, though for a long time they 
 were regarded as distinct. 
 
 The phenomenon of attraction and repulsion by 
 amber, which originated the term electricity, is as 
 unlike that of the decomposition of water by the 
 voltaic pile, as any two natural phenomena can 
 well be. It is only because the historical sequence 
 of scientific discoveries has associated them by a 
 number of intermediate links, that they are classed 
 under the same category. What is called voltaic 
 electricity might equally, perhaps more appropri- 
 ately, be called volta'c chemistry. I mention these 
 facts to show that the distinction in the name may 
 frequently be much greater than the distinction of 
 the subject which it represents, and vice vers&, not 
 as at all objecting to the received nomenclature on 
 these points ; nor do I say it would be advisable to 
 depart from it : were we to do so, inevitable confu- 
 sion would result, and objections equally forcible 
 might be found to apply to our new terminology. 
 
 Words, when established to a certain point, 
 become a part of the social mind ; its powers and 
 R
 
 242 CORRELATION OF PHYSICAL FORCES. 
 
 very existence depend upon the adoption of con- 
 ventional symbols ; and were these suddenly de- 
 parted from, or varied, according to individual 
 apprehensions, the acquisition and transmission of 
 knowledge would cease. Undoubtedly, neology is 
 more permissible in physical science than in any 
 other branch of knowledge, because it is more pro- 
 gressive ; new facts or new relations require new 
 names, but even here it should be used with great 
 
 caution. 
 
 Si forte necesse est 
 
 Indiciis monstrare recentibus abdita rerum, 
 Fingere cinctutis non exaudita Cethegis 
 Continget ; dabiturque licentia, sumpta pudenter. 
 
 Even should the mind ever be led to dismiss 
 the idea of various forces, and regard them as the 
 exertion of one force, or resolve them definitely 
 into motion ; still we could never avoid the use of 
 different conventional terms for the different modes 
 of action of this one pervading force. 
 
 Reviewing the series of relations between the 
 various forces which we have been considering, it 
 would appear that in many cases where one of these 
 is excited or exists, all the others are also set in 
 action : thus, when a substance, such as sulphuret 
 of antimony, is electrified, at the instant of elec- 
 trisation it becomes magnetic in directions at right 
 angles to the lines of electric force ; at the same 
 time it becomes heated to an extent greater or less 
 according to the intensity of the electric force. If
 
 CONCLUDING REMARKS. 243 
 
 this intensity be exalted to a certain point the 
 sulphuret becomes luminous, or light is produced : 
 it expands, consequently motion is produced; and 
 it is decomposed, therefore chemical action is pro- 
 duced. If we take another substance, say a metal, 
 all these forces except the last are developed ; and 
 although we can scarcely apply the term chemical 
 action to a substance hitherto undecomposed, and 
 which, under the circumstances we are considering, 
 enters into no new combination, yet a metal under- 
 goes that species of polarisation which, as far as we 
 can judge, is the first step towards chemical action, 
 and which, if the substance were decomposable, 
 would resolve it into its elements. Perhaps, indeed, 
 some hitherto undiscovered chemical action is pro- 
 duced in substances which we regard as undecom- 
 posable : there are experiments to show that metals 
 which have been electrised are permanently changed 
 in their molecular constitution. Oxygen, we have 
 seen, is changed by the electric spark into ozone, 
 and phosphorus into allotropic phosphorus, both 
 which changes were for a long time unknown to 
 those familiar with electrical science. 
 
 Thus, with some substances, when one mode of 
 force is produced all the others are simultaneously 
 developed. With other substances, probably with 
 all matter, some of the other forces are developed, 
 whenever one is excited, and all may be so were 
 the matter in a suitable condition for their develope- 
 & 2
 
 244r CORRELATION OF PHYSICAL FORCES. 
 
 racnt, or our means of detecting them sufficiently 
 delicate. 
 
 This simultaneous production of several diiferent 
 forces seems at first sight to be irreconcileable 
 with their mutual and necessary dependence, and 
 it certainly presents a formidable experimental 
 difficulty in the way of establishing their equi- 
 valent relations; but when examined closely, it is 
 not in fact inconsistent with the views we have 
 been considering, but is indeed a strong argument 
 in favour of the theory which regards them as 
 modes of motion. 
 
 Let us select one or two cases in which this 
 form of objection may be prominently put forward. 
 A voltaic battery decomposing water in a volta- 
 meter, while the same current is employed at the 
 same time to make an electro-magnet, gives never- 
 theless in the voltameter an equivalent of gas, or 
 decomposes an equivalent of an electrolyte for each 
 equivalent of chemical decomposition in the battery 
 cells, and will give the same ratios if the electro- 
 magnet be removed. Here, at first sight, it would 
 appear that the magnetism was an extra force pro- 
 duced, and that thus more than the equivalent 
 power was obtained from the battery. In answer 
 to this objection it may be said, that in the circum- 
 stances under which this experiment is ordinarily 
 performed, several cells of the battery are used, 
 and so there is a far greater amount of force gene-
 
 CONCLUDING REMARKS. 24.5 
 
 rated in the cells than is indicated by the effect in 
 the voltameter. If, moreover, the magnet be not 
 interposed, still the magnetic force is equally ex- 
 istent throughout the whole current ; for instance, 
 the wires joining the plates will attract iron filings, 
 deflect magnetic needles, &c., and produce diamag- 
 netic effects on surrounding matter. By the iron 
 core a small portion of the force is, indeed, absorbed 
 white it is being made a magnet, but this ceases 
 to be absorbed when the magnet is made ; this has 
 been proved by the observation of Mr. Latlmer 
 Clarke, who has found that along the wires of the 
 electric telegraph the magnetic needles placed 
 at different stations remained fixed after the con- 
 nection with the battery was made, and while the 
 electric current acted by induction on surrounding 
 conducting matter, separated from the wires by 
 their gutta percha coating, so that a sort of Ley den 
 phial was formed ; but as soon as this induction had 
 produced its effect between each station, or, so to 
 speak, the phial was charged, the needles successively 
 were deflected : it is like the case of a pulley and 
 weight, which latter exhausts force while it is 
 being raised ; but when raised, the force is free, and 
 may be used for other purposes. 
 
 If a battery of one cell, just capable of decom- 
 posing water and no more, be employed, this will 
 cease to decompose while making a magnet. There 
 jnnt. in every case, be preponderating chemical
 
 246 CORRELATION OF PHYSICAL FORCES 
 
 affinity in the battery cells, either by the nature of 
 its elements or by the reduplication of series, to 
 effect decomposition in the voltameter ; and if the 
 point is just reached at which this is effected, 
 and the power is then reduced by any resist- 
 ance, decomposition ceases : were it otherwise, were 
 the decomposition in the voltameter the expo- 
 nent of the entire force of the generating cells, 
 and these could independently produce magnetic 
 force, this latter force would be got from nothing, 
 and perpetual motion be obtained. 
 
 To take another and different example : A piece 
 of zinc dissolved in dilute sulphuric acid gives 
 somewhat less heat than when the zinc has a wire 
 of platinum attached to it, and is dissolved by the 
 same quantity of acid. The argument is deducible 
 that, as there is more electricity in the second 
 than in the first case, there should be less heat; 
 but as, according to our received theories, the heat 
 is a product of the electric current, and in con- 
 sequence of the impurity of zinc electricity is 
 generated in the first case molecularly, by what is 
 called local action, though not thrown into a general 
 direction, there should be more of both heat and 
 electricity in the second than in the first case, as 
 the heat and electricity due to the voltaic combina- 
 tion of zinc and platinum are added to that excited 
 on the surface of the zinc, and the zinc should be, 
 as in fact it is, more rapidly dissolved ; so that the
 
 CONCLUDING REMARKS $47 
 
 extra heat and electricity are produced by extra 
 chemical force. Many additional cases of a similar 
 description might be suggested. But although 
 it is difficult, and perhaps impossible, to restrict 
 the action of any one force to the production of 
 one other force, and of one only yet if the 
 whole of one force, say chemical action, be 
 supposed to be employed in producing its full 
 equivalent of another force, say heat, then as 
 this heat is capable in its turn of reproducing 
 chemical action, and in the limit, a quantity equal 
 or at least only infinitely short of the initial force : 
 if this could at the same tune produce indepen- 
 dently another force, say magnetism, we could, by 
 adding the magnetism to the total heat, get more 
 than the original chemical action, and thus create 
 force or obtain perpetual motion. 
 
 The term Correlation, which I selected as the 
 title of my Lectures in 1843, strictly interpreted, 
 means a necessary mutual or reciprocal dependence 
 of two ideas, inseparable even in mental concep- 
 tion : thus, the idea of height cannot exist without 
 involving the idea of its correlate, depth ; the idea 
 of parent cannot exist without involving the idea 
 of offspring. It has been scarcely, if at all, used 
 by writers on physics, but there are a vast variety 
 of physical relations to which, if it does not in its 
 strictest original sense apply, cannot certainly be 
 so well expressed by any other term. There are,
 
 248 CORRELATION OF PHYSICAL FORCES. 
 
 for example, many facts, one of which cannot take 
 place without involving the other ; one arm of a 
 lever cannot be depressed without the other being 
 
 elevated the finger cannot press the table without 
 
 the table pressing the finger. A body cannot be 
 heated without another being cooled, or some other 
 force being exhausted in an equivalent ratio to the 
 production of heat; a body cannot be positively 
 electrified without some other body being nega- 
 tively electrified, &c. 
 
 The probability is, that, if not all, the greater 
 number of physical phenomena are correlative, 
 and that, without a duality of conception, the 
 mind cannot form an idea of them: thus motion 
 cannot be perceived or probably imagined without 
 parallax or relative change of position. The world 
 was believed fixed, until, by comparison with the 
 celestial bodies, it was found to change its place 
 with regard to them: had there been no percep- 
 tible matter external to the world, we should never 
 have discovered its motion. In sailing along a 
 river, the stationary vessels and objects on the 
 banks seem to move past the observer: if at last 
 he arrives at the conviction that he is moving, and 
 not these objects, it is by correcting his senses by 
 reflection derived from a more extensive previous 
 use of them : even then he can only form a notion 
 of the motion of the vessel he is in, by its change 
 of position with regard to the objects it passes
 
 CONCLUDING REMARKS. 249 
 
 that is, provided his body partakes of the motion 
 of the vessel, which it only does when its course is 
 perfectly smooth, otherwise the relative change of 
 position of the different parts of the body and the 
 vessel inform him of its alternating, though not of 
 its progressive movement. So in all physical phe- 
 nomena, the effects produced by motion are all in 
 proportion to the relative motion: thus, whether 
 the rubber of an electrical machine be stationary, 
 and the cylinder mobile, or the rubber mobile and 
 the cylinder stationary, or both mobile in different 
 directions, or in the same direction with different 
 degrees of velocity, the electrical effects are, cceteris 
 paribus, precisely the same, provided the relative 
 motion is the same, and so, without exception, of 
 all other phenomena. The question of whether 
 there can be absolute motion, or, indeed, any abso- 
 lute isolated force, is purely the metaphysical 
 question of idealism or realism a question for our 
 purpose of little import; sufficient for the purely 
 physical inquirer, the maxim ' de non apparentibus 
 et non existentibus eadem est ratio' 
 
 The sense I have attached to the word correla- 
 tion, in treating of physical phenomena, will, I 
 think, be evident from the previous parts of this 
 essay, to be that of a necessary reciprocal produc- 
 tion ; in other words, that any force capable of pro- 
 ducing another may, in its turn, be produced by 
 it nav, more, can be itself resisted bv the force
 
 $50 CORRELATION OF PHYSICAL FORCES. 
 
 it produces, in proportion to the energy of such 
 production, as action is ever accompanied and re- 
 sisted by reaction : thus, the action of an electro- 
 magnetic machine is reacted upon by the magneto- 
 electricity developed by its action. 
 
 To many, however, of the cases we have been 
 considering, the term correlation may be applied in 
 a more strict accordance with its original sense: 
 thus, with regard to the forces of electricity and 
 magnetism in a dynamic state, we cannot electrise 
 a substance without magnetising it we cannot 
 magnetise it without electrising it: each mole- 
 cule, the instant it is affected by one of these forces, 
 is affected by the other; but, in transverse direc- 
 tions, the forces are inseparable and mutually de- 
 pendent correlative, but not identical. 
 
 The evolution of one force or mode of force into 
 another lias induced many to regard all the differ- 
 ent natural agencies as reducible to unity, and as 
 resulting from one force which is the efficient 
 cause of all the others : thus, one author writes to 
 prove that electricity is the cause of every change 
 in matter; another, that chemical action is the 
 cause of everything ; another, that heat is the uni- 
 versal cause, and so on. If, as I have stated it, 
 the true expression of the fact is, that each mode 
 of force is capable of producing the others, and that 
 none of them can be produced but by some other 
 as an anterior force, then any view which regards
 
 CONCLUDING REMARKS. 251 
 
 either of them as abstractedly the efficient cause of 
 all the rest, is erroneous : the view has, I believe, 
 arisen from a confusion between the abstract or 
 generalised meaning of the term cause, and its 
 concrete or special sense ; the word itself being 
 indiscriminately used in both these senses. 
 
 Another confusion of terms has arisen, and has, 
 indeed, much embarrassed me in enunciating the 
 propositions put forth in these pages, on account 
 of the imperfection of scientific language ; an im- 
 perfection in great measure unavoidable, it is true, 
 but not the less embarrassing. Thus, the words 
 light, heat, electricity, and magnetism, are con- 
 stantly used in two senses viz. that of the force 
 producing, or the subjective idea of force or power, 
 and of the effect produced, or the objective phe- 
 nomenon. The word motion, indeed, is only applied 
 to the effect, and not to the force, and the term 
 chemical affinity is generally applied to the force, 
 and not to the effect ; but the other four terms are, 
 for want of a distinct terminology, applied indis- 
 criminately to both. 
 
 I may have occasionally used the same word at 
 one time in a subjective, at another in an objective 
 sense ; all I can say is, that this cannot be avoided 
 without a neology, which I have not the presump- 
 tion to introduce, or the authority to enforce. 
 Again, the use of the term forces in the plural 
 might be objected to by those who do not attach
 
 252 CORRELATION OF PHYSICAL FORCES. 
 
 to the term force the notion of a specific agency, 
 but of one universal power associated with matter, 
 of which its various phenomena are but diversely 
 modified effects. 
 
 Whether the imponderable agents, viewed as 
 force, and not as matter, ought to be regarded as 
 distinct forces or as distinct modes of force, is pro- 
 bably not very material, for, as far as I am aware, 
 the same result would follow either view ; I have 
 therefore used the terms indiscriminately, as either 
 happened to be the more expressive for the 
 occasion. 
 
 Throughout this essay I have placed motion in 
 the same category as the other affections of matter. 
 The course of reasoning adopted in it, however, 
 appears to me to lead inevitably to the conclusion 
 that these affections of matter are themselves 
 modes of motion ; that, as in the case of friction, 
 the gross or palpable motion, which is arrested by 
 the contact of another body, is subdivided into 
 molecular motions or vibrations, which vibrations 
 are heat or electricity as the case may be; so 
 the other affections are only matter moved or 
 
 J 
 
 molecularly agitated in certain definite directions. 
 I have already considered the hypothesis that 
 the passage of electricity and magnetism causes 
 vibrations in an ether permeating the bodies 
 through which the current is transmitted, or the 
 application of the same ethereal hypothesis to these
 
 CONCLUDING REMARKS. 253 
 
 imponderables which had previously been applied 
 to light ; many, in speaking of some of their effect s, 
 admit that electricity and magnetism cause or pro- 
 duce by their passage vibrations in the particles of 
 matter, but regard the vibrations produced as an 
 occasional, though not always a necessary, effect of 
 the passage of electricity, or of the increment or 
 decrement of magnetism. The view which I have 
 taken is, that such vibrations, molecular polari- 
 sations, or motions of some sort from particle to 
 particle, are themselves electricity or magnetism ; 
 or, to express it in the converse, that dynamic elec- 
 tricity and magnetism are themselves motion, and 
 that permanent magnetism, and static electricity, 
 are conditions of force bearing a similar relation to 
 motion which tension or gravitation do. 
 
 This theory might well be discussed in greater 
 detail than has been used in this work ; but to do 
 this and to anticipate objections would lead into 
 specialities foreign to my present object, in the 
 course of this essay my principal aim having been 
 rather to show the relation of forces as evinced by 
 acknowledged facts, than to enter upon any detailed 
 explanation of their specific modes of action. 
 
 Probably man will never know the ultimate 
 structure of matter or the minutias of molecular 
 actions ; indeed it is scarcely conceivable that the 
 mind can ever attain to this knowledge ; the monad 
 irresolvable bv a given microscope may be resolved
 
 254 COERELATION OF PHYSICAL FORCES. 
 
 by an increase in power. Much harm has already 
 been done by attempting hypothetically to dissect 
 matter and to discuss the shapes, sizes, and num- 
 bers of atoms, and their atmospheres of heat, ether, 
 or electricity. 
 
 Whether the regarding electricity, light, mag- 
 netism, &c., as simply motions of ordinary matter, 
 be or be not admissible, certain it is, that all past 
 theories have resolved, and all existing theories do 
 resolve, the actions of these forces into motion. 
 Whether it be that, on account of our familiarity with 
 motion, we refer other affections to it, as to alanguage 
 which is most easily construed and most capable of 
 explaining them, or whether it be that it is in reality 
 the only mode in which our minds, as contradis- 
 tinguished from our senses, are able to conceive 
 material agencies ; certain it is, that since the period 
 at which the mystic notions of spiritual or preter- 
 natural powers were applied to account for physical 
 phenomena, all hypotheses framed to explain them 
 have resolved them into motion. Take, for ex- 
 ample, the theories of light to which I have before 
 alluded : one of these supposes light to be a highly 
 rare matter, emitted from i.e. put in motion by 
 luminous bodies ; a second supposes that the 
 matter is not emitted from luminous bodies, but 
 that it is put into a state of vibration or undulation, 
 i.e. motion, by them; and thirdly, light may be 
 regarded as an undulation or motion of ordinary
 
 CONCLUDING REMARKS. 255 
 
 matter, and propagated by undulations of air, glass, 
 &c., as I have before stated. In all these hypo- 
 theses, matter and motion are the only conceptions. 
 Nor, even if we accept terms derived from our own 
 sensations, the which sensations themselves may be 
 but modes of motion in the nervous filaments, can 
 we find words to describe phenomena other than 
 those expressive of matter and motion. We in vain 
 struggle to escape from these ideas ; if we ever do 
 so, our mental powers must undergo a change of 
 which at present we see no prospect. 
 
 If we apply to any other force the mode of 
 reasoning which we have in the course of this essay 
 applied to heat, we shall arrive at the same con- 
 clusion, and see that a given source of power can, 
 supposing it to be fully utilised in each case, yield 
 no more by employing it as an exciter of one force 
 than -of another. Let us take electricity as an 
 example. Suppose a pound of mercury at 400 be 
 employed to produce a thermo-electric current, and 
 the latter be in its turn employed to produce 
 mechanical force ; if this latter force be greater than 
 that which the direct effect of heat would produce, 
 then it could by compression raise the tempera- 
 ture of the mercury itself, or of a similar quantity 
 equally heated, to a higher point than its original 
 temperature, the 400 to 401, for example, which 
 is obviously impossible; nor, if we admit force 
 to be indestructible, can it produce less than 400,
 
 256 CORRELATION OF PHYSICAL FORCES. 
 
 except by some portion of it being converted into 
 another form or mode of force. 
 
 But as the mechanical effect here is produced 
 through the medium of electricity, and the me- 
 chanical effect is definite, so the quantity of elec- 
 tricity producing it must be definite also, for un- 
 equal quantities of electricity could only produce an 
 equal mechanical effect by a loss or gain of their 
 own force into or out of nothing. The same reason- 
 ing will apply to the other forces, and will lead, it 
 appears to me, necessarily and inevitably to the 
 conclusion, that each force is definitely and equi- 
 valently convertible into any other, and that 
 where experiment does not give the full equivalent, 
 it is because the initial force has been dissipated, 
 not lost, by conversion into other unrecognised 
 forces. The equivalent is the limit never practi- 
 cally reached. 
 
 The great problem which remains to be solved, 
 in regard to the correlation of physical forces, is 
 this establishment of their equivalents of power, 
 or their measurable relation to a given standard. 
 The progress made in some of the branches of 
 this inquiiy has been already noticed. Viewed in 
 their static relations, or in the conditions requisite 
 for producing equilibrium or quantitative equality 
 of force, a remarkable relation between chemical 
 affinity and heat is that discovered in many simple 
 bodies by Dulong and Petit, and extended to
 
 CONCLUDING REMAKES. 257 
 
 compounds by Neumann and Avogadro. Their 
 researches have shown that the specific heats of 
 certain substances, when multiplied by their che- 
 mical equivalents, give a constant quantity as 
 product or, in other words, that the combining 
 weights of such substances are those weights 
 which require equal accessions or abstractions of 
 heat, equally to raise or lower their temperature. 
 To put the proposition more in accordance with 
 the view we have taken of the nature of heat: 
 each body has a power of communicating or re- 
 ceiving molecular repulsive power, exactly equal, 
 weight for weight, to its chemical or combining 
 power. For instance, the equivalent of lead is 
 104, of zinc 33, or, in round numbers, as 3 to 1 : 
 these numbers are therefore inversely the ex- 
 ponents of their chemical power, three times as 
 much lead as zinc being required to saturate the 
 same quantity of an acid or substance combining 
 with it ; but their power of communicating or 
 abstracting heat or repulsive power is precisely 
 the same, for three times as much lead as zinc is 
 required to produce the same amount of expan- 
 sion or contraction in a given quantity of a third 
 substance, such as water. 
 
 Again, a great number of bodies chemically 
 combine in equal volumes, i.e. in the ratios of 
 their specific gravities ; but the specific gravities 
 represent the attractive powers of the substances,
 
 258 CORRELATION OF PHYSICAL FORCES. 
 
 or are the numerical exponents of the forces 
 tending to produce motion in masses of matter 
 towards each other ; while the chemical equivalents 
 are the exponents of the affinities or tendencies 
 of the molecules of dissimilar substances to com- 
 bine, and saturate each other ; consequently, here 
 we have in certain cases an equivalent relation 
 between these two modes of force gravitation 
 and chemical attraction. 
 
 Were the above relations extended into an uni- 
 versal law, we should have the same numerical 
 expression for the three forces of heat, gravity, 
 and affinity ; and as electricity and magnetism are 
 quantitatively related to them, we should have a 
 similar expression for these forces : but at present 
 the bodies in which this parity of force has been 
 discovered, though in themselves numerous, are 
 small compared with the exceptions, and, therefore, 
 this point can only be indicated as promising a 
 generalisation, should subsequent researches alter 
 our knowledge as to the elements and combining 
 equivalents of matter. 
 
 With regard to what may be called dynamic 
 equivalents, i.e. the definite relation to time of 
 the action of these varied forces upon equivalents 
 of matter, the difficulty of establishing them is 
 still greater. If the proposition which I stated at 
 the commencement of this paper be correct, that 
 motion may be subdivided or changed in character,
 
 CONCLUDING REMAEKS. 259 
 
 so as to become heat, electricity, &c., it ought to fol- 
 low that when we collect the dissipated and changed 
 forces, and reconvert them, the initial motion, minus 
 an infinitesimal quantity, affecting the same amount 
 of matter with the same velocity, should be repro- 
 duced, and so of the changes in matter produced by 
 the other forces ; but the difficulties of proving the 
 truth of this by experiment will, in many cases, be all 
 but insuperable ; we cannot imprison motion as we 
 can matter, though we may to some extent restrain 
 its direction. 
 
 The term perpetual motion, which I have not 
 unfrequently employed in these pages, is itself 
 equivocal. If the doctrines here advanced be 
 founded, all motion is, in one sense, perpetual. 
 In masses whose motion is stopped by mutual con- 
 cussion, heat or motion of the particles is generated ; 
 and thus the motion continues, so that if we could 
 venture to extend such thoughts to the universe, 
 we should assume the same amount of motion affect- 
 ing the same amount of matter for ever. Where 
 force opposes force, as in cases of static equilibrium, 
 the balance of pre-existing equilibrium is affected, 
 and fresh motion is started equivalent to that which 
 is withdrawn into a state of abeyance. 
 
 But the term perpetual motion is applied, in 
 
 ordinary parlance (and in such sense I have used 
 
 it), to a perpetual recurrent motion, e.g., a weight 
 
 which by its fall would turn a wheel, which wheel 
 
 9
 
 260 COKRELATION OF PHYSICAL FORCES. 
 
 would, in its turn, raise the initial weight, and so 
 on for ever, or until the material of which the 
 machine is made be worn out. It is strange that 
 to common apprehension the impossibility of this 
 is not self-evident : if the initial weight is to be 
 raised by the force it has itself generated, it must 
 necessarily generate a force greater than that of 
 its own weight or centripetal attraction ; in other 
 words, it must be capable of raising a weight 
 heavier than itself : so that, setting aside the resist- 
 ance of friction, &c., a weight, to produce perpetual 
 recurrent motion, must be heavier than an equal 
 weight of matter, in short, heavier than itself. 
 
 Suppose two equal weights at each end of an 
 equi-armed lever, there is no motion ; cut off a 
 fraction of one of them, and it rises while the other 
 falls. How, now, is the lesser weight to bring back 
 the greater without any extraneous application of 
 force? If, as is obvious, it cannot do so in this simple 
 form of experiment, it is a fortiori more impossible 
 if machinery be added, for increased resistances 
 have then to be overcome. Can we again mend 
 this by employing any other force? Suppose we 
 employ electricity, the initial weight in descending 
 turns a cylinder against a cushion, and so generates 
 electricity ; to make this force recurrent, the elec- 
 tricity so generated must, in its turn, raise the 
 initial weight, or one heavier than it, i.e. the initial 
 weight must, through the medium of the electricity
 
 CONCLUDING REMARKS. 261 
 
 it generates, raise a weight heavier than itself. The 
 same problem, applied to any other forces, will 
 involve the same absurdity : and yet, simple as the 
 matter seems, the world is hardly yet disabused of 
 an idea little removed from superstition. 
 
 But the importance of the deductions to be de- 
 rived from the negation of perpetual motion seems 
 scarcely to have impressed philosophers, and we 
 only find here and there a scattered hint of the 
 consequences necessarily resulting from that which 
 to the thinking mind is a conviction. Some of 
 these I have ventured to put forward in the present 
 essay, but many remain, and will crowd upon the 
 mind of those who pursue the subject. Does not, 
 for instance, the impossibility of perpetual motion, 
 when thought out, involve the demonstration of 
 the impossibility, to which I have previously alluded, 
 of any event identically recurring ? 
 
 The pendulum in vacuo, at each beat, leaves a 
 portion of the force which started it in the form of 
 heat at its point of suspension ; this force, though 
 ever existent, can never be restored in its integrity 
 to the ball of the pendulum, for in the process of 
 restoration it must affect other matter, and alter 
 the condition of the universe. To restore the initial 
 force in its integrity, everything as it existed at the 
 moment of the first beat of the pendulum must be 
 restored in its integrity : but how can this be for 
 while the force was escaping from the pendulum
 
 262 CORRELATION OF PHYSICAL FORCES. 
 
 by radiating heat from the point of suspension, 
 surrounding matter has not stood still; the very 
 attraction which caused the beat of the pendulum 
 has changed in degree, for the pendulum is nearer 
 to or farther from the sun, or from some planet or 
 fixed star. 
 
 It might be an interesting and not profitless 
 speculation to follow out these and other conse- 
 quences ; it would, I believe, lead us to the convic- 
 tion that the universe is ever changing, and that 
 notwithstanding secular recurrences which would 
 prima facie seem to replace matter in its original 
 position, nothing in fact ever returns or can return 
 to a state of existence identical with a previous state. 
 But the field is too illimitable for me to venture 
 farther. 
 
 The inevitable dissipation or throwing off a 
 portion of the initial force presents a great ex- 
 perimental difficulty in the way of establishing the 
 equivalents of the various natural forces. In the 
 steam-engine, for instance, the heat of the furnace 
 not only expands the water and thereby produces 
 the motion of the piston, but it also expands the 
 iron of the boiler of the cylinder and all surround- 
 ing bodies. The force expended in expanding this 
 iron to a very small extent is equal to that which 
 expands the vapour to a very large extent: this 
 expansion of the iron is capable, in its turn, of 
 producing a great mechanical force, which is
 
 CONCLUDING EEMARKS. 263 
 
 practically lost. Could all the force be applied to 
 the vapour, an enormous addition of power would 
 be gained for the same expenditure; and perhaps 
 even with our present means more might be done 
 in utilising the expansion of the iron. 
 
 Another great difficulty in experimentally ascer- 
 taining the dynamic equivalents of different forces 
 arises from the effects of disruption, or the over- 
 coming an existing force. Thus, when a part of the 
 initial force employed is engaged in twisting or 
 tearing asunder matter previously held together by 
 cohesive attraction, or in overcoming gravitation or 
 inertia, the same amount of heat or electricity 
 would not apparently be evolved as if such ob- 
 stacle were non-existent, and the initial force were 
 wholly employed in producing, not in opposing. 
 There is great difficulty in devising experiments 
 in which some portion of the force is not so 
 employed. 
 
 The initial force, however, that has been employed 
 for such disruption is not lost, as at the moment of 
 disruption the bodies producing it fly off, and carry 
 with them their force. Thus, let two weights be 
 attached to a cord placed across a bar; when their 
 force is sufficient to break the cord or the bar, 
 the weights fall and strike the earth, making it 
 vibrate, and so conveying away or continuing the 
 force expressed by the cohesion of the bar or cord. 
 If, instead of breaking a cord, the weights be
 
 264 CORRELATION OF PHYSICAL FORCES. 
 
 employed to bend a bar, their gravitating force, 
 instead of making the earth vibrate, produces heat 
 in the bar, and so with whatever other force be 
 employed to produce effects of disruption, torsion, 
 &c., so that, though difficult in practice, the nu- 
 merical problem of the equivalent of the force is 
 not theoretically irresolvable. 
 
 The voltaic battery affords us the best means of 
 ascertaining the dynamic equivalents of different 
 forces, and it is probable that by its aid the best 
 theoretical and practical results will be ultimately 
 attained. 
 
 In investigating the relation of the different 
 forces, I have in turn taken each one as the initial 
 force or starting-point, and endeavoured to show 
 how the force thus arbitrarily selected could medi- 
 ately or immediately produce and be merged into 
 the others: but it will be obvious to those who 
 have attentively considered the subject, and brought 
 their minds into a general accordance with the 
 views I have submitted to them, that no force can, 
 strictly speaking, be initial, as there must be some 
 anterior force which produced it : we cannot create 
 force or motion any more than we can create matter. 
 
 Thus, to take an example previously noticed, and 
 recede backwards; the spark of light is produced 
 by electricity, electricity by motion, and motion is 
 produced by something else, say a steam-engine 
 that is, by heat. This heat is produced by chemical
 
 CONCLUDING REMAEKS. 265 
 
 affinity, i. e. the affinity of the carbon of the 
 coal for the oxygen of the air: this carbon and 
 this oxygen have been previously eliminated by 
 actions difficult to trace, but of the pre-existence 
 of which we cannot doubt, and in which actions 
 we should find the conjoint and alternating effects 
 of heat, light, chemical affinity, &c. Thus, tracing 
 any force backwards to its antecedents, we are 
 merged in an infinity of changing forms of force ; 
 at some point we lose it, not because it has been in 
 fact created at any definite point, but because it 
 resolves itself into so many contributing forces, 
 that the evidence of it is lost to our senses or 
 powers of detection; just as, in following it forward 
 into the effect it produces, it becomes, as I have 
 before stated, so subdivided and dissipated as to be 
 equally lost to our means of detection. 
 
 Can we, indeed, suggest a proposition, definitely 
 conceivable by the mind, of force without antece- 
 dent force? I cannot, without calling for the in- 
 terposition of creative power, any more than I can 
 conceive the sudden appearance of a mass of matter 
 come from nowhere, and formed from nothing. 
 The impossibility, humanly speaking, of creating 
 or annihilating matter, has long been admitted, 
 though, perhaps, its distinct reception in philosophy 
 may be set down to the overthrow of the doctrine 
 of Phlogiston, and the reformation of chemistry 
 at the time of Lavoisier. The reasons for the
 
 266 CORRELATION OF PHYSICAL FORCES. 
 
 admission of a similar doctrine as to force appear to 
 be equally strong. With regard to matter, there 
 are many cases in which we never practically prove 
 its cessation of existence, yet we do not the less 
 believe in it : who, for instance, can trace, so as to 
 re-weigh, the particles of iron worn off the tire of 
 a carriage wheel? who can recombine the particles 
 of wax dissipated and chemically changed in the 
 burning of a candle ? By placing matter under- 
 going physical or chemical changes under special 
 limiting circumstances, we may, indeed, acquire 
 evidence of its continued existence, weight for 
 weight and so we may in some instances of 
 force, as in definite electrolysis: indeed, the evi- 
 dence we acquire of the continued existence of 
 matter is by the continued exertion of the force 
 it exercises, as, when we weigh it, our evidence is 
 the force of attraction; so, again, our evidence 
 of force is the matter it acts upon. Thus, matter 
 and force are correlates, in the strictest sense of 
 the word; the conception of the existence of the 
 one involves the conception of the existence of the 
 other : the quantity of matter again, and the degree 
 of force, involve conceptions of space and time. 
 But to follow out these abstract relations would 
 lead me too far into the alluring paths of meta- 
 physical speculation. 
 
 That the theoretical portions of this essay are 
 open to objection I am fully conscious. I cannot,
 
 CONCLUDING EEMAEKS. 267 
 
 however, but think that the fair way to test a 
 theory is to compare it with other theories, and to 
 see whether upon the whole the balance of proba- 
 bility is in its favour. Were a theory open to no 
 objection it would cease to be a theory, and would 
 become a law ; and were we not to theorise, or to 
 take generalised views of natural phenomena until 
 those generalisations were sure and unobjectionable 
 in other words, were laws science would be 
 lost in a complex mass of unconnected observa- 
 tions, which would probably never disentangle 
 themselves. Excess on either side is to be avoided ; 
 although we may often err on the side of hasty 
 generalisation, we may equally err on the side of 
 mere elaborate collection of observations, which, 
 though sometimes leading to a valuable result, yet, 
 when cumulated without a connecting link, fre- 
 quently occasion a costly waste of time, and leave 
 the subject to which they refer in greater obscurity 
 than that in which it was involved at their com- 
 mencement. 
 
 Collections of facts differ in importance, as do 
 theories: the former, in many instances, derive 
 their value from their capability of generalisation ; 
 while, conversely, theories are valuable as methods 
 of co-ordinating given series of facts, and more 
 valuable in proportion as they require fewer excep- 
 tions and fewer postulates. Facts may sometimes 
 be as well explained by one view as by another,
 
 268 CORKELATION OF PHYSICAL FORCES. 
 
 but without a theory they are unintelligible and 
 incommunicable. Let us use our utmost effort to 
 communicate a fact without using the language 
 of theory, and we fail; theory is involved in all 
 our expressions ; the knowledge of bygone times is 
 imported into succeeding times by terms involving 
 theoretic conceptions. As the knowledge of any 
 particular science developes itself, our views of it 
 become more simple; hypotheses, or the introduc- 
 tion of supposititious views, are more and more 
 dispensed with ; words become applicable more 
 directly to the phenomena, and, losing the hypo- 
 thetic meaning which they necessarily possessed at 
 their reception, acquire a secondary sense, which 
 brings more immediately to our minds the facts of 
 which they are indices. The scaffolding has served 
 its purpose. The hypothesis fades away, and a 
 theory, or generalised view of phenomena, more in- 
 dependent of supposition, but still full of gaps and 
 difficulties, takes its place. This in its turn, should 
 the science continue to progress, either gives place 
 to a more simple and wider generalisation, or be- 
 comes, by the removal of objections, established as 
 a law. Even in this more advanced stage words 
 importing theory must be used, but phenomena are 
 now intelligible and connected, though expressed 
 by varied forms of speech. 
 
 To think on nature is to theorise ; and difficult 
 it is not to be led on by the continuities of natural
 
 CONCLUDING KEMARKS. 269 
 
 phenomena to theories which appear forced and 
 unintelligible to those who have not pursued the 
 same path of thought ; which, moreover, if allowed 
 to gain an undue influence, seduce us from that 
 truth which is the sole object of our pursuit. 
 
 Where to draw the line where to say thus far 
 we may go, and no farther, in any particular class 
 of analogies or relations which Nature presents to 
 us; how far to follow the progressive indications 
 of thought, and where to resist its allurements 
 is a question of degree which must depend upon 
 the judgement of each individual or of each class 
 of thinkers; yet it is consolatory that thought is 
 seldom expended in vain. 
 
 I have throughout endeavoured to discard the 
 hypotheses of subtle or occult entities; if in this 
 endeavour some of my views have been adopted 
 upon insufficient data, I still hope that this essay 
 will not prove valueless. 
 
 The conviction that the so-called imponderables 
 are modes of motion, will, at all events, lead the 
 observer of natural phenomena to look for changes 
 in these affections, wherever the intimate structure 
 of matter is changed ; and, conversely, to seek for 
 changes in matter, either temporary or permanent, 
 whenever it is affected by these forces. I believe 
 he will seldom do this in vain. It was not until I 
 had long reflected on the subject, that I ventured 
 to publish my views ; their publication may induce
 
 270 CORRELATION OF PHYSICAL FORCES. 
 
 others to think on their subject-matter. They are 
 not put forward with the same objects, nor do they 
 aim at the same elaboration of detail, as memoirs 
 on newly-discovered physical facts: they purport 
 to be a method of mentally regarding known facts, 
 some few of which I have myself made known on 
 other occasions, but the great mass of which have 
 been accumulated by the labours of others, and are 
 admitted as established truths. Every one has a 
 right to view these facts through any medium he 
 thinks fit to employ, but some theory must exist 
 in the minds of those who reflect upon the many 
 new phenomena which have recently, and more 
 particularly during the present century, been dis- 
 covered. It is by a generalised or connected view 
 of past acquisitions in natural knowledge that 
 deductions can best be drawn as to the probable 
 character of the results to be anticipated. It is 
 a great assistance in such investigations to be 
 intimately convinced that no physical phenomenon 
 can stand alone : each is inevitably connected with 
 anterior changes, and as inevitably productive of 
 consequential changes, each with the other, and all 
 with time and space; and, either in tracing back 
 these antecedents or following up their conse- 
 quents, many new phenomena will be discovered, 
 and many existing phenomena hitherto believed 
 distinct will be connected and explained : explana- 
 tion is, indeed, only relation to something more
 
 CONCLUDING EEMAKKS. 271 
 
 familiar, not more known i.e. known as to causa- 
 tive or creative agencies. In all phenomena the 
 more closely they are investigated the more are we 
 convinced that, humanly speaking, neither matter 
 nor force can be created or annihilated, and that 
 
 an essential cause is unattainable Causation is 
 
 the will, Creation the act, of God.
 
 272 
 
 ADDKESS 
 
 OP 
 
 WILLIAM EGBERT GROVE, ESQ. 
 
 PRESIDENT OF THB BRITISH ASSOCIATION FOB THE ADVANCEMENT 
 OF SCIENCE, NOTTINGHAM, 1866. 
 
 IF our rude predecessors, who at one time inhabited 
 the caverns which surround this town, could rise 
 from their graves and see it in its present state, it 
 may be doubtful whether they would have sufficient 
 knowledge to be surprised. 
 
 The machinery, almost resembling organic beings 
 in delicacy of structure, by which are fabricated 
 products of world-wide reputation, the powers of 
 matter applied to give motion to that machinery, 
 are so far removed from what must have been the 
 conceptions of the semi-barbarians to whom I have 
 alluded, that they could not look on them with in- 
 telligent wonder. 
 
 Yet this immense progress has all been effected 
 step by step, now and then a little more rapidly
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 273 
 
 than at other times ; but, viewing the whole course 
 of improvement, it has been gradual, though mov- 
 ing in an accelerated ratio. But it is not merely 
 in those branches of natural knowledge which tend 
 to improvements in economical arts and manufac- 
 tures, that science has made great progress. In the 
 study of our own planet and the organic beings 
 with which it is crowded, aud in so much of the 
 universe as vision, aided by the telescope, has 
 brought within the area of observation, the pre- 
 sent century has surpassed any antecedent period 
 of equal duration. 
 
 It would be difficult to trace out all the causes 
 which have led to the increase of observational and 
 experimental knowledge. 
 
 Among the more thinking portion of mankind 
 the gratification felt by the discovery of new truths, 
 the expansion of faculties, and extension of the 
 boundaries of knowledge, have been doubtless a 
 sufficient inducement to the study of nature; 
 while, to more practical minds, the reality, the cer- 
 tainty, and the progressive character of the acqui- 
 sitions of natural science, and the enormously 
 increased means whieh its applications give, have 
 impressed its importance as a minister to daily 
 wants and a contributor to ever-increasing material 
 comforts, luxury, and power. 
 
 Though by no means the only one, yet an im- 
 portant cause of the rapid advance of science is the 
 T
 
 274 CONTINUITY. 
 
 growth of associations for promoting the progress 
 either of physical knowledge generally, or of special 
 branches of it. Since the foundation of the Royal 
 Society, now more than two centuries ago, a vast 
 number of kindred societies have sprung up in this 
 country and in Europe. The advantages conferred 
 by these societies are manifold ; they enable those 
 who are devoted to scientific research, to combine, 
 compare, and check their observations, to assist, by 
 the thoughts of several minds, the promotion of the 
 inquiry undertaken ; they contribute from a joint 
 purse to such efforts as their members deem most 
 worthy; they afford a means of submitting to a 
 competent tribunal notices and memoirs, and of 
 obtaining for their authors and others, by means of 
 the discussions which ensue, information given by 
 those best informed on the particular subject; they 
 enable the author to judge whether it is worth his 
 while to pursue the subjects he has brought for- 
 ward, and they defray the expense of printing and 
 publishing such researches as are thought meri- 
 torious. 
 
 These advantages, and others might be named, 
 pertain to the Association the thirty-sixth meeting of 
 which we are this evening assembled to inaugurate; 
 but it has, from its intermittent and peripatetic 
 character, advantages which belong to none of the 
 societies which are fixed as to their locality. 
 
 Among these are the novelty and freshness of an
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 275 
 
 annual meeting, which, while it brings together old 
 members of the Association, many of whom only 
 meet on this occasion, always adds a quota of new 
 members, infusing new blood, and varying the social 
 character of our meetings. 
 
 The visits of distinguished foreigners, whom we 
 have previously known by reputation, is one of the 
 most delightful and improving of the results. The 
 wide field of inquiry, and the character of com- 
 munications made to the Association, including all 
 branches of natural knowledge, and varying from 
 simple notices of an interesting observation or ex- 
 periment, to the most intricate and refined branches 
 of scientific research, is another valuable charac- 
 teristic. 
 
 Lastly, perhaps, the greatest advantage resulting 
 from the annual visits of this great parliament to 
 new localities is that, while it imparts fresh local 
 knowledge to the visitors, it leaves behind stimu- 
 lating memories, which rouse into permanent acti- 
 vity dormant or timid minds an effect which, so 
 far from ceasing with the visit of the Association, 
 frequently begins when that visit terminates. 
 
 Every votary of physical science must be anxious 
 to see it recognised by those institutions of his 
 country which can to the greatest degree promote 
 its cultivation and reap from it the greatest benefit. 
 You will probably agree with me that the principal 
 educational establishments on the one hand, and on 
 1 2
 
 276 CONTINUITY. 
 
 the other the Government, in many of its depart- 
 ments, are the institutions which may best fulfil 
 these conditions. The more early the mind is 
 trained to a pursuit of any kind, the deeper and 
 more permanent are the impressions received, and 
 the more service can be rendered by the students. 
 
 ' Quo semel est imbuta recens, senrabit odorem 
 Testa diu.' 
 
 Little can be achieved in scientific research without 
 an acquaintance with it in youth ; you will rarely 
 find an instance of a man who has attained any 
 eminence in science who has not commenced its 
 study at a very early period of life. Nothing, 
 again, can tend more to the promotion of science 
 than the exertions of those who have early acquired 
 the fag resulting from a scientific education. I 
 desire to make no complaint of the tardiness with 
 which science has been received at our public 
 schools, and, with some exceptions, at our univer- 
 sities. These great establishments have their roots 
 in historical periods, and long time and patient 
 endeavour is requisite before a new branch of 
 thought can be grafted with success on a stem to 
 which it is exotic. Nor should I ever wish to see 
 the study of languages, of history, of all those 
 refined associations which the past has transmitted 
 to us, neglected ; but there is room for both. It is 
 sad to see the number of so-called educated men
 
 ADDEESS TO THE BRITISH ASSOCIATION, 1866. 277 
 
 who, travelling by railway, voyaging by steamboat, 
 consulting the almanac for the time of sunrise or 
 full-moon, have not the most elementary knowledge 
 of a steam-engine, a barometer, or a quadrant ; and 
 who will listen with a half-confessed faith to the 
 most idle predictions as to weather or cometic in- 
 fluences, while they are in a state of crass ignorance 
 as to the cause of the trade winds, or the form of a 
 comet's path. May we hope that the slight infil- 
 tration of scientific studies, now happily commenced, 
 will extend till it occupies its fair space in the 
 education of the young, and that those who may be 
 able learnedly to discourse on the Eolic digamma 
 will not be ashamed of knowing the principles on 
 which the action of an air-pump, an electrical 
 machine, or a telescope, depends, and will not, as 
 Bacon complained of his contemporaries, despise 
 such knowledge as something mean and mechanical. 
 To assert that the great departments of Govern- 
 ment should encourage physical science may appear 
 a truism, and yet it is but of late that it has been 
 seriously done ; now, the habit of consulting men 
 of science on important questions of national in- 
 terest is becoming a recognised practice, and in a 
 time, which may seem long to individuals, but is 
 short in the history of a nation, a more definite 
 sphere of usefulness for national purposes will, I 
 have no doubt, be provided for those duly qualified 
 men who may be content to give up the more
 
 278 CONTINUITY. 
 
 tempting study of abstract science for that of its 
 practical applications'. In this respect the report 
 of the Kew Committee for this year affords a 
 subject of congratulation to those whom I have 
 the honour to address. The Kew Observatory, the 
 petted child of the British Association, may pos- 
 sibly become an important national establishment ; 
 and if so, while it will not, I trust, lose its character 
 of a home for untrammelled physical research, it 
 will have superadded some of the functions of the 
 Meteorological Department of the Board of Trade, 
 with a staff of skilful and experienced observers. 
 
 This is one of the results which the general 
 growth of science, and the labours of this Associa- 
 tion in particular, have produced; but I do not 
 propose on this occasion to recapitulate the special 
 objects attained by the Association ; this has been 
 amply done by several of my predecessors; nor 
 shall I confine my address to the progress made in 
 physical science since the time when my most able 
 and esteemed friend arid predecessor addressed you 
 at Birmingham. In the various reports and com- 
 munications which will be read at your sections, 
 details of every step which has been made in 
 science since our last meeting will be brought to 
 your notice, and I have no doubt fully and freely 
 discussed. 
 
 I purpose, with your kind permission, to submit 
 to you certain views of what has within a com-
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 279 
 
 paratively recent period been accomplished by 
 science, what have been the steps leading to the 
 attained results, and what, as far as we may fairly 
 form an opinion, is the general character pervading 
 modern discovery. 
 
 It seems to me that the object we have in view 
 would be more nearly approached, by each presi- 
 dent, chosen as they are in succession as repre- 
 senting different branches of science, giving on 
 these occasions either an account of the progress of 
 the particular branch of science he has cultivated, 
 when that is not of a very limited and special 
 character, or enouncing his own view of the general 
 progress of science; and though this will neces- 
 sarily involve much that belongs to recent years, 
 the confining a president to a mere resume of what 
 has taken place since our last meeting would, I 
 venture with diffidence to think, limit his means 
 of usefulness, and render his discourse rather an 
 annual register than an instructive essay. 
 
 I need not dwell on the commonplace but yet 
 important topics of the material advantages re- 
 sulting from the application of science ; I will 
 address myself to what, in my humble judgement, 
 are the lessons we have learned, and the probable 
 prospects of improved natural knowledge. 
 
 One word will give you the key to what I am 
 about to discourse on ; that word is continuity, no 
 new word, and used in no new sense, but perhaps
 
 280 CONTINUITY. 
 
 applied more generally than it has hitherto been. 
 We shall see, unless I am much mistaken, that the 
 developement of observational, experimental, and 
 even deductive knowledge is either attained by 
 steps so extremely small as to form really a con- 
 tinuous ascent ; or, when distinct results apparently 
 separate from any co-ordinate phenomena have been 
 attained, that then, by the subsequent progress of 
 science, intermediate link's have been discovered 
 uniting the apparently segregated instances with 
 other more familiar phenomena. We shall see 
 that the more we investigate, the more we find 
 that in existing phenomena graduation from the 
 like to the seemingly unlike prevails, and in the 
 changes which take place in time, gradual progress 
 is, and apparently must be, the course of nature. 
 
 Let me now endeavour to apply this view to the 
 recent progress of some of the .more prominent 
 branches of science. 
 
 In Astronomy, from the time when the earth 
 was considered a flat plain bounded by a flat ocean 
 when the sun, moon, and stars were regarded as 
 lanterns to illuminate this plain each successive 
 discovery has brought with it similitudes and 
 analogies between this earth and many of the 
 objects of the universe, with which our senses, 
 aided by instruments, have made us acquainted. 
 I pass, of course, over those discoveries which have 
 established the Copernican system as applied to our
 
 ADDEESS TO THE BRITISH ASSOCIATION, 1866. 281 
 
 sun, its attendant planets, and their satellites. The 
 proofs, however, that gravitation is not confined to 
 our solar system, but pervades the universe, have 
 received many confirmations by the labours of 
 members of this Association ; I may name those 
 who have held the office of President, Lord Rosse, 
 Lord Wrottesley, and Sir J. Herschel, the two 
 latter having devoted special attention to the orbits 
 of double stars, the former to those probably more 
 recent systems called nebulae. Double stars seem 
 to be orbs analogous to our own sun and revolving 
 round their common centre of gravity in a conic- 
 section curve, as do the planets with which we are 
 more intimately acquainted ; but the nebulaB pre- 
 sent more difficulty, and some doubt has been ex- 
 pressed whether gravitation, such as we consider 
 it, acts with those bodies (at least those exhibiting 
 a spiral form) as it does with us; possibly some 
 other modifying influence may exist, our present 
 ignorance of which gives rise to the apparent 
 difficulty. There is, however, another class of ob- 
 servations quite recent in its importance, and which 
 has formed a special subject of contribution to the 
 reports and transactions of this Association ; I al- 
 lude to those on Meteorites, at which our lamented 
 member, and to many of us our valued friend, Prof. 
 Baden Powell, assiduously laboured, for investiga- 
 tions into which a committee of this Association is 
 formed, and a series of star-charts for enabling
 
 282 CONTINUITY. 
 
 observers of shooting-stars to record their observa- 
 tions was laid before the last meeting of the As- 
 sociation by Mr. Glaisher. 
 
 It would occupy too much of your time to detail 
 the efforts of Bessel, Schwirike, the late Sir J. 
 Lubbock, and others, as applied to the formation 
 of star-charts for aiding the observation of meteor- 
 ites which Mr. Alexander Herschel, Mr. Brayley, 
 Mr. Sorby, and others are now studying. 
 
 Dr. Olmsted explained the appearance of a point 
 from which the lines of flight of meteors seem to 
 radiate, as being the perspective vanishing point of 
 their parallel or nearly parallel courses appearing 
 to an observer on the earth as they approach it. 
 The uniformity of position of these radiant points, 
 the many corroborative observations on the direc- 
 tion, the distances, and the velocities of these 
 bodies, the circumstance that their paths intersect 
 the earth's orbit at certain definite periods, and 
 the total failure of all other theories which have 
 been advanced, while there is no substantial ob- 
 jection to this, afford evidence almost amounting to 
 proof that these are cosmical bodies moving in the 
 interplanetary space by gravitation round the sun, 
 and some perhaps round planets. This view gives 
 us a new element of continuity. The universe 
 would thus appear not to have the extent of empty 
 space formerly attributed to it, but to be studded 
 between the larger and more visible masses with
 
 ADDEESS TO THE BRITISH ASSOCIATION, 1866. 283 
 
 smaller planets, if the term be permitted to be 
 applied to meteorites. 
 
 Observations are now made at the periods at 
 which meteors appear in greatest numbers at 
 Greenwich by Mr. Glaisher, at Cambridge by Prof. 
 Adams, and at Hawkhurst by Mr. Alexander 
 Herschel and every preparation is made to secure 
 as much accuracy as can, in the present state of 
 knowledge, be secured for such observations. 
 
 The number of known asteroids, or bodies of a 
 smaller size than what are termed the ancient 
 planets, has been so increased by numerous dis- 
 coveries, that instead of seven we now count 
 eighty-eight as the number of recognised planets 
 a field of discovery with which the name of 
 Hind mil be ever associated. 
 
 The smallest of these is only twenty or thirty 
 miles in diameter, indeed cannot be accurately 
 measured, and if we were to apply the same 
 scrutiny to other parts of the heavens as has been 
 applied to the zone between Mars and Jupiter, 
 it is no far-fetched speculation to suppose that, 
 in addition to asteroids and meteorites, many 
 other bodies exist until the space occupied by our 
 solar system becomes filled up with planetary bodies 
 varying in size from that of Jupiter (1240 times 
 larger in volume than the earth) to that of a 
 cannon-ball or even a pistol-bullet. 
 
 The researches of Leverrier on the intra-mer-
 
 284 CONTINUITY. 
 
 curial planets come in aid of these views ; and 
 another half-century may, and not improbably 
 will, enable us to ascertain that the now seem- 
 ingly vacant interplanetary spaces are occupied by 
 smaller bodies which have hitherto escaped ob- 
 servation, just as the asteroids had until the time 
 of Olbers and Piazzi. But the evidence of con- 
 tinuity as pervading the universe does not stop 
 at telescopic observation ; chemistry and physical 
 optics bring us new proofs. Those meteoric bodies 
 which have from time to time come so far within 
 reach of the earth's attraction as to fall upon its 
 surface, give on analysis metals and oxides similar 
 to those which belong to the structure of the earth 
 they come as travellers bringing specimens of 
 minerals from extra-terrestrial regions. 
 
 In a series of papers recently communicated to 
 the French Academy, M. Daubre"e has discussed 
 the chemical and mineralogical character of me- 
 teorites as compared with the rocks of the earth. 
 He finds that the similarity of terrestrial rocks to 
 meteorites increases as we penetrate deeper into 
 the earth's crust, and that some of the deep-seated 
 minerals have a composition and characteristics 
 almost identical with meteorites [olivine, herzolite, 
 and serpentine, for instance, closely resemble 
 them] ; that as we approach the surface, rocks 
 having similar components with meteorites are 
 found, but in a state of oxidation, which neces-
 
 ADDRESS TO THE BEITISH ASSOCIATION, 1866. 285 
 
 sarily much modifies their mineral character, and 
 which, by involving secondary oxygenised com- 
 pounds, must also change their chemical constitu- 
 tion. By experiments he has succeeded in form- 
 ing from terrestrial rocks substances very much 
 resembling meteorites. Thus close relationship, 
 though by no means identity, is established be- 
 tween this earth and those wanderers from remote 
 regions, some evidence, though at present incom- 
 plete, of a common origin. 
 
 Surprise has often been expressed that, while 
 the mean specific gravity of this globe is from five 
 to six times that of water, the mean specific 
 gravity of its crust is barely half as great. It has 
 long seemed to me that there is no ground for 
 wonder here. The exterior of our planet is to a 
 considerable depth oxidated ; the interior is in all 
 probability free from oxygen, and whatever bodies 
 exist there are in a reduced or deoxidated state ; if 
 so, their specific gravity must necessarily be higher 
 than that of their oxides or chlorides, &c. ; we find, 
 moreover, that some of the deep-seated minerals 
 have a higher specific gravity than the average of 
 those on the surface; olivine, for instance, has a 
 specific gravity of 3 '3. There is therefore no a 
 priori improbability that the mean specific gravity 
 of the earth should notably exceed that of its 
 surface; and if we go further, and suppose the 
 interior of the earth to be formed of the same
 
 286 CONTINUITY. 
 
 ingredients as the exterior, minus oxygen, chlorine, 
 bromine, &c., a specific gravity of 5 to 6 would 
 not be an unlikely one. Many of the elementary 
 bodies entering largely into the formation of the 
 earth's crust are as light or lighter than water 
 for instance, potassium, sodium, &c. ; others, such 
 as sulphur, silicon, aluminium, have from two to 
 three times its specific gravity; others, again, 
 as iron, copper, zinc, tin, seven to nine times ; 
 while others, lead, gold, platinum, &c., are much 
 more dense but, speaking generally, the. more 
 dense are the least numerous. There seems no 
 improbability in a mixture of such substances pro- 
 ducing a mean specific gravity of from 5 to 6, 
 although it by no means follows, indeed the pro- 
 bability is rather the other way, that the pro- 
 portions of the substances in the interior of the 
 earth are the same as on the exterior. It mid it 
 be worth the labour to ascertain the mean specific 
 gravity of all the known minerals on the earth's 
 surface, averaging them in the ratios in which, M< 
 far as our knowledge goes, they quantitatively 
 exist, and assuming them to exist without the 
 oxygen, chlorine, &c., with which they are, with 
 some rare exceptions, invariably combined on the 
 surface of the earth : great assistance to the know- 
 ledge of the probable constitution of the earth 
 might be derived from such an investigation. 
 While chemistry, analytic and synthetic, thus
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 287 
 
 aids us in ascertaining the relationship of our 
 planet to meteorites, its relation in composition to 
 other planets, to the sun, and to more distant suns 
 and systems, is aided by another science, viz. optics. 
 
 That light passing from one transparent medium 
 to another should carry with it evidence of the 
 source from which it emanates, would, until lately, 
 have seemed an extravagant supposition ; but pro- 
 bably (could we read it) everything contains in 
 itself a large portion of its own history. 
 
 I need not detail to you the discoveries of 
 Kirchhoff, Bunsen, Miller, Huggins, and others; 
 they have been dilated on by my predecessor. 
 Assuming that spectrum analysis is a reliable 
 indication of the presence of given substances by 
 the position of -transverse bright lines exhibited 
 when they are burnt, and of transverse dark lines 
 when light is transmitted through their vapours, 
 though Pliicker has shown that with some sub- 
 stances these lines vary with temperature, the 
 point of importance in the view I am presenting 
 to you is, that while what may be called com- 
 paratively neighbouring cosmical bodies exhibit 
 lines identical with many of those shown by the 
 components of this planet, as we proceed to the 
 more distant appearances of the nebulae we get 
 but one or two of such lines, and we get one or 
 two new bands not yet identified with any known 
 to be produced by substances on this globe.
 
 288 CONTINUITY. 
 
 Within the last year Mr. Huggins has added to 
 his former researches observations on the spectrum 
 of a comet (comet 1 of 1866), the nucleus of 
 which shows but one bright line, while the spec- 
 trum formed by the light of the coma is continuous, 
 seeming to show that the nucleus is gaseous, while 
 the coma would consist of matter in a state of 
 minute division shining by reflected light : whether 
 this be solid, liquid, or gaseous is doubtful, but the 
 author thinks it is in a condition analogous to that 
 of fog or cloud. The position in the spectrum of 
 the bright line furnished by the nucleus is the 
 same as that of nitrogen, which line is also shown 
 in some of the nebulas. 
 
 But the most remarkable achievement by spec- 
 trum analysis is the record of observations on a 
 temporary star which has shone forth this year in 
 the constellation of the northern crown about a 
 degree S.E. of the star e. When it was first seen, 
 May 12, it was nearly equal in brilliancy to a star 
 of the second magnitude ; when observed by Mr. 
 Huggins and Dr. Miller, May 16, it was reduced 
 to the third or fourth magnitude. Examined by 
 these observers with the spectroscope, it gave a 
 spectrum which they state was unlike that of any 
 celestial body they had examined. 
 
 The light was compound and had emanated from 
 two different sources. One spectrum was analo- 
 gous to that of the sun, viz. formed by the light of
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 289 
 
 an incandescent solid or liquid photosphere which 
 had suffered absortion by the vapours of an enve- 
 lope cooler than itself. The second spectrum con- 
 sisted of a few bright lines, which indicated that 
 the light by which it was formed was emitted by 
 matter in the state of luminous gas. The observers 
 consider that, from the position of two of the bright 
 lines, the gas must be probably hydrogen, and from 
 their brilliancy compared with the light of the photo- 
 sphere the gas must have been at a very high tem- 
 perature. They imagine the phenomena to result 
 from the burning of hydrogen with some other 
 element, and that from the resulting temperature 
 the photosphere is heated to incandescence. 
 
 There is strong reason to believe that this star is 
 one previously seen by Argelander and Sir J. Her- 
 schel, and that it is a variable star of long or irre- 
 gular period; it is also notable that some of its 
 spectrum lines correspond with those of several va- 
 riable stars. The time of its appearance was too 
 short for any attempt to ascertain its parallax; it 
 would have been important if it could even have 
 been established that it is not a near neighbour, as 
 the magnitude of such a phenomenon must depend 
 upon its distance. I forbear to add any specula- 
 tions as to the cause of this most singular pheno- 
 menon ; however imperfect the knowledge given us 
 by these observations, it is a great triumph to have 
 u
 
 290 CONTINUITY. 
 
 caught this fleeting object, and obtained permanent 
 records for the use of future observers. 
 
 It would seem as if the phenomenon of gradual 
 change obtained towards the remotest objects with 
 which we are at present acquainted, and that the 
 further we penetrate into space the more unlike to 
 those we are acquainted with become the objects of 
 our examination sun, planets, meteorites, earth 
 similarly though not identically constituted, stars 
 differing from each other and from our system, and 
 nebula? more remote in space and differing, more in 
 their characters and constitution. 
 
 While we can thus to some extent investigate 
 the physical constitution of the most remote visible 
 substances, may we not hope that some further 
 insight as to the constitution of the nearest, viz. 
 our own satellite, may be given us by this class of 
 researches? The question whether the moon pos- 
 sesses any atmosphere may still be regarded as 
 unsolved. If there be any, it must be exceedingly 
 small in quantity and highly attenuated. Calcula- 
 tions, made from occultations of stars, on the appa- 
 rent differences of the semidiameter of the bright 
 and dark moon give an amount of difference which 
 might indicate a minute atmosphere, but which 
 Mr. Airy attributes to irradiation. 
 
 Supposing the moon to be constituted of similar 
 materials to the earth, it must be, to say the least, 
 doubtful whether there is oxygen enough to oxidate 

 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 291 
 
 the metals of which she is composed; and if not, 
 the surface which we see must be metallic, or nearly 
 so. The appearance of her craters is not unlike 
 that seen on the surface of some metals, such as 
 bismuth, or, according to Professor Phillips, silver, 
 when cooling from fusion and just previous to soli- 
 difying; and it might be a fair subject of enquiry 
 whether, if there be any coating of oxide on the sur- 
 face, it may not be so thin as not to disguise the form 
 of the congealed metallic masses, as they may have 
 set in cooling from igneous fusion. M. Chacornac's 
 recent observations lead him to suppose that many 
 of the lunar craters were the result of a single 
 explosion, which raised the surface as a bubble and 
 deposited its debris around the orifice of eruption. 
 The eruptions on the surface of the moon clearly 
 did not take place at one period only, for at many 
 parts of the disk craters may be seen encroaching 
 on and disfiguring more ancient craters, sometimes 
 to the extent of three or four successive displace- 
 ments : two important questions might, it seems to 
 me, be solved by an attentive examination of such 
 portions of the moon. By observing carefully with 
 the most powerful telescopes the character of the 
 ridges thus successively formed, the successive 
 states of the lunar surface at different epochs might 
 be elucidated ; and secondly, as on the earth we 
 should look for actual volcanic action at those 
 points where recent eruptions have taken place, 
 u 2
 
 29-2 CONTINUITY. 
 
 so on the moon the more recently active points 
 being ascertained by the successive displace- 
 ment of anterior formations, it is these points 
 which should be examined for existing disruptive 
 disturbances. Metius and Fabricius might be 
 cited as points of this character, having been 
 found by M. Chacornac to present successive dis- 
 placements and to be perforated by numerous chan- 
 nels or cavities. M. Chacornac considers that the 
 seas, as they are called, or smoother portions of the 
 lunar surface have at some time made inroads on 
 anteriorly formed craters ; if so, a large portion of 
 the surface of the moon must have been in a fused, 
 liquid, semiliquid, or alluvial state long after the 
 solidifying of other portions of it. It would be 
 difficult to suppose that this state was one of ig- 
 neous fusion, for this could hardly exist over a 
 large part of the surface without melting up the 
 remaining parts; on the other hand, the total ab- 
 sence of any signs of water, and of any, or, if anv, 
 only the most attenuated, atmosphere, would make 
 it equally difficult to account for a large diluvhil 
 formation. 
 
 Some substances, like mercury on this planet, 
 might have remained liquid after others had solidi- 
 fied ; but the problem is one which needs more 
 examination and study before any positive opinion 
 can be pronounced. 
 
 I cannot pass from the subject of lunar physics
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 293 
 
 without recording the obligation we are under to 
 our late President for his most valuable observa- 
 tions and for his exertion in organising a band of 
 observers devoted to the examination of this our 
 nearest celestial neighbour, and to Mr. Nasmyth 
 and Mr. De la Rue for their important graphical 
 and photographical contributions to this sub- 
 ject. The granular character of the sun's sur- 
 face observed by Mr. Nasmyth in 1860 is also 
 a discovery which ought not to be passed over in 
 silence. 
 
 Before quitting the subject of astronomy I can- 
 not avoid expressing a feeling of disappointment 
 that the achromatic telescope, which has rendered 
 such notable service to this science, still retains in 
 practice the great defect which was known a cen- 
 tury ago at the time of Hall and Dollond, namely, 
 the inaccuracy of definition arising from what was 
 termed the irrationality of the spectrum, or the 
 incommensurate divisions of the spectra formed by 
 flint and crown glass. 
 
 The beautiful results obtained by Blair have 
 remained inoperative from the circumstance that 
 evaporable liquids being employed between the 
 lenses, a want of permanent uniformity in the 
 instrument was experienced ; and notwithstanding 
 the high degree of perfection to which the grinding 
 and polishing object-glasses has been brought by 
 Clarke, Cooke, and Mertz, notwithstanding the
 
 294 CONTINUITY. 
 
 greatly improved instrumental manufacture, the 
 defect to which I have adverted remains unre- 
 medied and an eyesore to the observer with the 
 refracting telescope. 
 
 We have now a large variety of different kinds 
 of glass formed from different metallic oxides. A 
 list of many such was given by M. Jacquelain a 
 few years back; the last specimen which I have 
 seen is a heavy high refracting glass formed from 
 the metal thallium by M. Lamy. Among all these, 
 could not two or three be selected which, having 
 appropriate refracting and dispersing powers, would 
 have the coloured spaces of their respective spectra 
 if not absolutely in the same proportions, at all 
 events much more nearly so than those of flint 
 and crown glass ? Could not, again, oily or 
 resinous substances having much action on the 
 more refrangible rays of the spectrum, such as 
 castor oil, Canada balsam, &c., be made use of in 
 combination with glass lenses to reduce if not 
 annihilate this signal defect? This is not a pro- 
 blem to the solution of which there seems any 
 insuperable difficulty; the reason why it has not 
 been solved is, I incline to think, that the great 
 practical opticians have no time at their disposal to 
 devote to long tentative experiments and calcula- 
 tions, and on the other hand the theoretic opticians 
 have not the machinery and the skill in manipula- 
 tion requisite to give the appropriate degree of
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 295 
 
 excellence to the materials with which they experi- 
 ment; yet the result is worth labouring for, as, 
 could the defect be remedied, the refracting tele- 
 scope would make nearly as great an advance upon 
 its present state as the achromatic did on the single 
 lens refractor. 
 
 While gravitation, physical constitution, and 
 chemical analysis by the spectrum show us that 
 matter has similar characteristics in other worlds 
 than our own, when we pass to the consideration of 
 those other attributes of matter which were at one 
 time supposed to be peculiar kinds of matter itself, 
 or, as they were called, imponderables, but which 
 are now generally, if not universally, recognised as 
 forces or modes of motion, we find the evidence of 
 continuity still stronger. 
 
 When all that was known of magnetism was that a 
 piece of steel rubbed against a particular mineral had 
 the power of attracting iron, and, if freely suspended, 
 of arranging itself nearly in a line with the earth's 
 meridian, it seemed an exceptional phenomenon. 
 When it was observed that amber, if rubbed, had 
 the temporary power of attracting light bodies, this 
 also seemed something peculiar and anomalous. 
 What are now magnetism and electricity ? forces 
 so universal, so apparently connected with matter 
 as to become two of its invariable attributes, and 
 that to speak of matter not being capable of being 
 affected by these forces would seem almost as
 
 296 CONTINUITY. 
 
 extravagant as to speak of matter not being affected 
 by gravitation. 
 
 So with light, heat, and chemical affinity, not 
 merely is every form of matter with which we are 
 acquainted capable of manifesting all these modes 
 of force, but so-called matter supposed incapable of 
 such manifestations would to most minds cease to 
 be matter. 
 
 Further than this it seems to me (though, as I 
 have taken an active part for many years, now 
 dating from a quarter of a century, in promoting 
 this view, I may not be considered an impnrti:il 
 judge) that it is now proved that all these forces 
 are so invariably connected inter se and with 
 motion as to be regarded as modifications of each 
 other, and as resolving themselves objectively 
 into motion, and subjectively into that something 
 which produces or resists motion, and which we 
 call force. 
 
 I may perhaps be permitted to recal a forgotten 
 experiment, which nearly a quarter of a century 
 ago I showed at the London Institution, an experi- 
 ment simple enough in itself, but which then 
 seemed to me important from the consequences to 
 be deduced from it, and the importance of which 
 will be much better appreciated now than then. 
 
 A train of multiplying wheels ended with a small 
 metallic wheel which, when the train was put in 
 motion, revolved with extreme rapidity against the
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 297 
 
 periphery of the next wheel, a wooden one. In the 
 metallic wheel was placed a small piece of phos- 
 phorus, and as long as the wheels revolved the 
 phosphorus remained unchanged, but the moment 
 the last wheel was stopped, by moving a small lever 
 attached to it, the phosphorus burst into flame. 
 My object was to show that while motion of the mass 
 continued, heat was not generated, but that when 
 this was arrested, the force continuing to operate, 
 the motion of the mass became heat in the particles. 
 The experiment differed from that of Rumforcl's 
 cannon-boring and Davy's friction of ice in show- 
 ing that there was no heat while the motion was 
 unresisted, but that the heat was dependent on 
 the motion being impeded or arrested. We have 
 now become so accustomed to this view, that 
 whenever we find motion resisted we look to 
 heat, electricity, or some other force as the neces- 
 sary and inevitable result. 
 
 It would be out of place here, and treating of 
 matters too familiar to the bulk of my audience, to 
 trace how by the labours of Oersted, Seebeck, 
 Faraday. Talbot, Daguerre, and others, materials 
 have been provided for the generalisation now 
 known as the correlation of forces or conservation 
 of energy, while Davy, Rumford, Seguin, Mayer, 
 Joule, Helmholtz, Thomson, and others (among 
 whom I would not name myself, were it not that I 
 may be misunderstood and supposed to have aban-
 
 298 CONTINUITY. 
 
 doned all claim to a share in the initiation of this, 
 as I believe, important generalisation) have carried 
 on the work ; and how, sometimes by independent 
 and, as is commonly the case, nearly simultaneous 
 deductions, sometimes by progressive and accumu- 
 lated discoveries, the doctrine of the reciprocal in- 
 teraction, of the quantitative relation, and of the 
 necessary dependence of all the forces has, I think 
 I may venture to say, been established. 
 
 If magnetism be, as it is proved to be, connected 
 with the other forces or affections of matter, if 
 electrical currents always produce, as they are 
 proved to do, lines of magnetic force at right 
 angles to their lines of action, magnetism must be 
 cosmical, for where there is heat and light there is 
 electricity, and consequently magnetism. Magne- 
 tism, then, must be cosmical and not merely terres- 
 trial. Could we trace magnetism in other planets 
 and suns as a force manifested in axial or meridional 
 lines, i. e. in lines cutting at right angles the curves 
 formed by their rotation round an axis, it would 
 be a great step ; but it is one hitherto unaccom- 
 plished. The apparent coincidences between the 
 maxima and minima of solar spots, and the decen- 
 nial or undecennial periods of terrestrial magnetic 
 intensity, though only empirical at present, might 
 tend to lead us to a knowledge of the connection 
 we are seeking; and the President of the Royal 
 Society considers that an additional epoch of coin-
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 299 
 
 cidence has arrived, making the fourth decennial 
 period ; but some doubt is thrown upon these coin- 
 cidences by the magnetic observations made at 
 Greenwich Observatory. In a paper published in 
 the ' Transactions of the Royal Society,' 1863, the 
 Astronomer Royal says, speaking of results extend- 
 ing over seventeen years, there is no appearance of 
 decennial cycle in the recurrence of great magnetic 
 disturbances; and Mr. Glaisher last year, in the 
 physical section of this Association, stated that 
 after persevering examination he had been unable 
 to trace any connection between the magnetism of 
 the earth and the spots on the sun. 
 
 Mr. Airy, however, in a more recent paper, sug- 
 gests that currents of magnetic force having refe- 
 rence to the solar hour are detected, and seem 
 to produce vortices or circular disturbances, and 
 he invites further cooperative observation on the 
 subject, one of the highest interest, but at present 
 remaining in great obscurity. 
 
 One of the most startling suggestions as to the 
 consequence resulting from the dynamical theory of 
 heat is that made by Mayer, that by the loss of vis 
 viva occasioned by friction of the tidal waves, as 
 well as by their forming, as it were, a drag upon 
 the earth's rotatory movement, the velocity of the 
 earth's rotation must be gradually diminishing, and 
 that thus, unless some undiscovered compensatory 
 action exist, this rotation must ultimately cease,
 
 300 CONTINUITY. 
 
 and changes hardly calculable take place in the 
 solar system. 
 
 M. Delaunay considers part of the acceleration 
 of the moon's mean motion which is not at pre- 
 sent accounted for by planetary disturbances, to 
 be due to the gradual retardation of the earth's 
 rotation ; to which view, after an elaborate inves- 
 tigation, the Astronomer Royal has given his 
 assent. 
 
 Another most interesting speculation of Mayer 
 is that with which you are familiar, viz. that the 
 heat of the sun is occasioned by friction or percus- 
 sion of meteorites falling upon it : there are some 
 difficulties, not perhaps insuperable, in this theory. 
 Supposing such cosmical bodies to exist in sufficient 
 numbers they would, as they revolve round the 
 sun, fall into it, not as an aerolite falls upon the 
 earth directly by an intersection of orbits, but by 
 the gradual reduction in size of the orbits, occa- 
 sioned by a resisting medium ; some portion of 
 force would be lost, and heat generated in space by 
 friction against such medium ; when they arrive at 
 the sun they would, assuming them, like the 
 planets, to have revolved in the same direction, all 
 impinge in a definite direction, and we might ex- 
 pect to see some symptoms of such in the sun's 
 photosphere ; but though this is in a constant state 
 of motion, and the direction of these movements 
 has been carefully investigated by Mr. Carrington'
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 301 
 
 and others, no such general direction is detected ; 
 and M. Faye, who some time ago wrote a paper 
 pointing out many objections to the theory of solar 
 heat being produced by the fall of meteoric bodies 
 into the sun, has recently investigated the proper 
 motions of sun spots, and believes he has removed 
 certain apparent anomalies and reduced their mo- 
 tions to a certain regularity in the motion of the 
 photosphere, attributable to some general action 
 arising from the internal mass of the sun. 
 
 It might be expected that comets, bodies so light 
 and so easily deflected from their course, would 
 show some symptoms of being acted on by gravi- 
 tation, were such a number of bodies to exist 
 in or near their paths as are presupposed in the 
 mechanical theory of solar heat. 
 
 Assuming the undulatory theory of light to be 
 true, and that the motion which constitutes light is 
 transmitted across the interplanetary spaces by a 
 highly elastic ether, then unless this motion is 
 confined to one direction, unless there be no inter- 
 ference, unless there be no viscosity, as it is now 
 termed, in the medium, and, consequently, no fric- 
 tion, light must lose something in its progress from 
 distant luminous bodies, that is to say, must lose 
 something as light ; for, as all reflecting minds are 
 now convinced that force cannot be annihilated, 
 the force is not lost, but its mode of action is 
 changed. If light, then, is lost as light (and the
 
 302 CONTINUITY. 
 
 observations of Struve seem to show this to be so, 
 that, in fact, a star may be so far distant that it 
 can never be seen in consequence of its luminous 
 emissions becoming extinct), what becomes of the 
 transmitted force lost as light, but existing in some 
 other form ? So with heat : our sun, our earth, 
 and planets are constantly radiating heat into space, 
 so in all probability are the other suns, the stars, 
 and their attendant planets. What becomes of the 
 heat thus radiated into space ? If the universe 
 have no limit, and it is difficult to conceive one, 
 heat and light should be everywhere uniform ; and 
 yet more is given off than is received by each 
 cosmical body, for otherwise night would be as 
 light and as warm as day. What becomes of the 
 enormous force thus apparently non-recurrent in 
 the same form ? Does it return as palpable mo- 
 tion ? Does it move or contribute to move suns 
 and planets ? And can it be conceived as a force 
 similar to that which Newton speculated on as 
 universally repulsive and capable of being sub- 
 stituted for universal attraction ? We are in no 
 position at present to answer such questions as 
 these ; but I know of no problem in celestial 
 dynamics more deeply interesting than this, and we 
 may be no further removed from its solution than 
 the predecessors of Newton were from the simple 
 dynamical relation of matter to matter which 
 that potent intellect detected and demonstrated.
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 303 
 
 Passing from extra terrestrial theories to the 
 narrower field of molecular physics, we find the 
 doctrine of correlation of forces steadily making 
 its way. In the Bakerian Lecture for 1863, Mr. 
 Sorby shows, not 'perhaps a direct correlation of 
 mechanical and chemical forces, but that when, 
 either by solution or by chemical action, a change 
 in volume of the resulting substance as compared 
 with that of its separate constituents is effected, 
 the action of pressure retards or promotes the 
 change, according as the substance formed would 
 occupy a larger or smaller space than that occupied 
 by its separate constituents ; the application of 
 these experiments to geological inquiries as to sub- 
 terranean changes which may have taken place 
 under great pressure is obvious, and we may ex- 
 pect to form compounds under artificial compression 
 which cannot be found under normal pressure. 
 
 In a practical point of view the power of con- 
 verting one mode of force into another is of the 
 highest importance, and with reference to a subject 
 which at present, somewhat prematurely, perhaps, 
 occupies men's minds, viz. the prospective exhaus- 
 tion of our coal-fields, there is every encouragement 
 derivable from the knowledge that we can at will 
 produce heat by the expenditure of other forces ; 
 but, more than that, we may probably be enabled 
 to absorb or store up, as it were, diffused energy 
 for instance, Berthelot has found that the potential
 
 304 CONTINUITY. 
 
 energy of formate of potash is much greater than 
 that of its proximate constituents, caustic potash 
 and carbonic oxide. This change may take place 
 spontaneously and at ordinary temperatures, and 
 by such change carbonic oxide becomes, so to 
 speak, reinvested with the amount of potential 
 energy which its carbon possessed before uniting 
 with oxygen, or, in other words, the carbonic oxide 
 is raised as a force-possessor to the place of carbon 
 by the direct absorption or conversion of heat from 
 surrounding matter. 
 
 Here we have as to force -absorption, an analogous 
 result to that of the formation of coal from car- 
 bonic acid and water ; and though this is a mere 
 illustration, and may never become economical on 
 a large scale, still it and similar examples may 
 calm apprehension as to future means of supplying 
 heat, should our present fuel become exhausted. 
 As the sun's force, spent in times long past, is now 
 returned to us from the coal which was formed by 
 that light and heat, so the sun's rays, which are 
 daily wasted, as far as we are concerned, on tlu; 
 sandy deserts of Africa, may hereafter, by chemical 
 or mechanical means, be made to light and warm 
 the habitations of the denizens of colder regions. 
 The tidal wave is, again, a large reservoir of force 
 hitherto almost unused. 
 
 The valuable researches of Professor Tyndall on 
 radiant heat, afford many instances of the power of
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 305 
 
 localising, if the term be permitted, heat which 
 would otherwise be dissipated. 
 
 The discoveries of Graham, by which atmo- 
 spheric air, drawn through films of caoutchouc, 
 leaves behind half its nitrogen, or, in other words, 
 becomes richer by half in oxygen, and hence has a 
 much increased potential energy, not only show a 
 most remarkable instance of physical molecular 
 action, merging into chemical, but afford us indica- 
 tions of means of storing up force, much of the 
 force used hi working the aspirator being capable 
 at any period, however remote, of being evolved 
 by burning the oxygen with a combustible. 
 
 What changes may take place in our modes of 
 applying force before the coal-fields are exhausted 
 it is impossible to predict. Even guesses at the 
 probable period of their exhaustion are uncertain. 
 There is a tendency to substitute for smelting in 
 metallurgic processes, liquid chemical action, which 
 of course has the effect of saving fuel ; and the 
 waste of fuel in ordinary operations is enormous, 
 and can be much economised by already known 
 processes. It is true that we are, at present, far 
 from seeing a practical mode of replacing that 
 granary of force the coal-fields ; but we may with 
 confidence rely on invention being in this case, as 
 in others, born of necessity, when the necessity 
 arises. 
 
 I will not further pursue this subject ; at a time 
 x
 
 306 CONTINUITY. 
 
 when science and civilisation cannot prevent large 
 tracts of country being irrigated by human blood 
 in order to gratify the ambition of a few restless 
 men, it seems an over-refined sensibility to occupy 
 ourselves with providing means for our descendants 
 in the tenth generation to warm their dwellings or 
 propel their locomotives. 
 
 Two very remarkable applications of the con- 
 vertibility of force have been recently attained by 
 the experiments of Mr. Wilde and Mr. Holz ; the 
 former finds that, by conveying electricity from the 
 coils of a magneto-electric machine to an electro- 
 magnet, a considerable increase of electrical powi-r 
 may be attained, and by applying this as a mag- 
 neto-electric machine to a second, and this in turn 
 to a third electro-magnetic apparatus, the force is 
 largely augmented. Of course, to produce this 
 increase, more mechanical force must be used at 
 each step to work the magneto-electric machines; 
 but provided this be supplied, there hardly seems a 
 limit to the extent to wliich mechanical may be 
 converted into electrical force. 
 
 Mr. Holz has contrived a Franklinic electrical 
 machine, in which a similar principle is manifested. 
 A varnished glass plate is made to revolve in close 
 proximity to another plate having two or more 
 pieces of card attached, which are electrified by 
 a bit of rubbed glass or ebonite ; the moment this 
 is effected, a resistance is felt by the operator who
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 307 
 
 turns the handle of the machine, and the slight 
 temporary electrization of the card converts into a 
 continuous flood of intense electricity the force 
 supplied by the arm of the operator. 
 
 These results offer great promise of extended 
 application ; they show that, by a mere formal dis- 
 position of matter, one force may be converted into 
 another, and that not to the limited extent hitherto 
 attained, but to an extent co-ordinate, or nearly so, 
 with the increased initial force, so that, by a mere 
 change in the arrangement of apparatus, a means 
 of absorbing and again eliminating in a new form 
 a given force may be obtained to an indefinite 
 extent. As we may, in a not very distant future, 
 need, for the daily uses of mankind, heat, light, 
 and mechanical force, and find our present re- 
 sources exhausted, the more we can invent new 
 modes of conversion of forces, the more prospect 
 we have of practically supplying such want. It is 
 but a month from this time that the greatest 
 triumph of force-conversion has been attained. 
 The chemical action generated by a little salt-water 
 on a few pieces of zinc will now enable us to con- 
 verse with inhabitants of the opposite hemisphere 
 of this planet, and 
 
 ' Put a girdle round about the earth in forty minutes.' 
 
 The Atlantic Telegraph is an accomplished fact, 
 In physiology very considerable strides are being 
 
 x 2
 
 308 CONTINUITY. 
 
 made by studying the relation of organised bodies 
 to external forces; and this branch of enquiry has 
 been promoted by the labours of Carpenter, Bence 
 Jones, Playfair, E. Smith, Frankland, and others. 
 Vegetables acted on by light and heat, decompose 
 water, ammonia, and carbonic acid, and transform 
 them into, among other substances, oxalate of lime, 
 lactic acid, starch, sugar, stearine, urea, and ulti- 
 mately albumen ; while the animal reverses the 
 process, as does vegetable decay, and produces from 
 albumen, urea, stearine, sugar, starch, lactic acid, 
 oxalate of lime, and ultimately ammonia, water, and 
 carbonic acid. 
 
 As, moreover, heat and light are absorbed, or 
 converted in forming the synthetic processes going 
 on in the vegetable, so conversely heat and some 
 times light is given off by the living animal ; but it 
 must not be forgotten that the line of demarcation 
 between a vegetable and an animal is difficult to 
 draw, that there are no single attributes which are 
 peculiar to either, and that it is only by a number 
 of characteristics that either can be defined. 
 
 The series of processes above given may be 
 simulated by the chemist in his laboratory ; and 
 the amount of labour which a man has undergone 
 in the course of twenty-four hours may be approxi- 
 mately arrived at by an examination of the chemical 
 changes which have taken place in his body ; changed 
 forms in matter indicating the anterior exercise of
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 309 
 
 dynamical force. That muscular action is pro- 
 duced or supported by chemical change would 
 probably now be a generally accepted doctrine; 
 but while many have thought that muscular power 
 is derived from the oxidation of albuminous or 
 nitrogenised substances, several recent researches 
 seem to show that the latter is rather an accom- 
 paniment than a cause of the former, and that it is 
 by the oxidation of carbon and hydrogen com- 
 pounds that muscular force is supplied. Traube 
 has been prominent in advancing this view, and 
 experiments detailed in a paper published this year 
 by two Swiss professors, Drs. Fick and Wislicenus, 
 which were made by and upon themselves in an 
 ascent of the Faulhorn, have gone far to confirm it. 
 Having fed themselves, before and during the 
 ascent, upon starch, fat, and sugar, avoiding all 
 nitrogenised compounds, they found that the con- 
 sumption of such food was amply sufficient to 
 supply the force necessary for their expedition, and 
 that they felt no exhaustion. By appropriate 
 chemical examination they ascertained that there 
 was no notable increase in the oxidation of the 
 nitrogenised constituents of the body. After cal- 
 culating the mechanical equivalents of the com- 
 bustion effected, they state, as their first con- 
 clusion, that ' the burning of protein substances 
 cannot be the only source of muscular power, for 
 we have here two cases in which men performed
 
 310 CONTINUITY. 
 
 more measurable work than the equivalent of the 
 amount of heat which, taken at a most absurdly 
 high figure, could be calculated to result from the 
 burning of the albumen.' 
 
 They further go on to state that, so far from the 
 oxidation of albuminous substances being the only 
 source of muscular power, ' the substances by the 
 burning of wliich force is generated in the muscles 
 are not the albuminous constituents of those tissues, 
 but non-nitrogenous substances, either fats or hy- 
 drates of carbon,' and that the burning of albumen 
 is not in any way concerned in the production of 
 muscular power. 
 
 We must not confuse the question of the food 
 which forms and repairs muscle and gives per- 
 manent capability of muscular force with that 
 which supplies the requisites for temporary ac- 
 tivity; no doubt the carnivora are the most power- 
 fully constituted animals, but the chamois, gazelle, 
 fec., have great temporary capacity for. muscular 
 exertion, though their food is vegetable; for con- 
 centrated and sustained energy, however, they do 
 not equal the carnivora ; and with the domestic 
 graminivora we certainly find that they are capable 
 of performing more continuous work when sup- 
 plied with those vegetables which contain the 
 greatest quantity of nitrogen. 
 
 These and many similar classes of research show 
 that in chemical enquiries, as in other branches of
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 311 
 
 science, we are gradually relieving ourselves of 
 hypothetical existences, which certainly had the 
 advantage that they might be varied to suit the 
 requirements of the theorist. 
 
 Phlogiston, as Lavoisier said with a sneer, was 
 sometimes heavy, sometimes light ; sometimes fire 
 in a free state, sometimes combined; sometimes 
 passing through glass vessels, sometimes retained 
 by them ; which by its protean changes explained 
 causticity and non-causticity, transparency and 
 opacity, colours and their absence. As phlogiston 
 and similar creations of the mind have passed 
 away, so with hypothetic fluids, imponderable 
 matters, specific ethers, and other inventions of 
 entities made to vary according to the require- 
 ments of the theorist, I believe the day is ap- 
 proaching when these will be dispensed with, and 
 when the two fundamental conceptions of matter 
 and motion will be found sufficient to explain 
 physical phenomena. 
 
 The facts made known to us by geological en- 
 quiries, while on the one hand they afford striking 
 evidence of continuity, on the other, by the breaks 
 in the record, may be used as arguments against 
 it. The great question once was, whether these 
 chasms represent sudden changes in the formation 
 of the earth's crust, or whether they arise from 
 dislocations occasioned since the original deposi- 
 tion of strata or from gradual shifting of the areas
 
 312 CONTINUITY. 
 
 of submergence. Few geologists of the present 
 day would, I imagine, not adopt the latter alterna- 
 tives. Then comes a second question, whether, 
 when the geological formation is of a continuous 
 character, the different characters of the fossils 
 represent absolutely permanent varieties, or may 
 be explained by gradual modifying changes. 
 
 Professor Ansted, summing up the evidence on 
 this head as applied to one division of stratified 
 rocks, writes as follows : ' Palaeontologists have 
 endeavoured to separate the Lias into a number of 
 subdivisions, by the Ammonites, groups of species 
 of those shells being characteristic of different 
 zones. The evidence on this point rests on the 
 assumption of specific differences being indicated 
 by permanent modifications of the structure of the 
 shell. But it is quite possible that these may 
 mean nothing more than would be due to some 
 change in the conditions of existence. Except 
 between the Marlstone and the Upper Lias there is 
 really no pataontological break, in the proper sense 
 of the words; alterations of form and size conse- 
 quent on the occurrence of circumstances more or 
 less favourable, migration of species, and other 
 well-known causes, sufficiently account for many 
 of those modifications of the form of the shell that 
 have been taken as specific marks. This view is 
 strengthened by the fact that other shells and 
 other organisms generally show no proof of a
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 313 
 
 break of any importance except at the point al- 
 ready alluded to.' 
 
 But, irrespectively of another deficiency in the 
 geological record, which will be noticed presently, 
 the physical breaks in the stratification make 
 it next to impossible to fairly trace the order of 
 succession of organisms by the evidence afforded 
 by their fossil remains. Thus there are nine 
 great breaks in the Palaeozoic series, four in the 
 Secondary, and one in the Tertiary, besides those 
 between Palaeozoic and Secondary and Secondary 
 and Tertiary respectively. Thus in England there 
 are sixteen important breaks in the succession of 
 strata, together with a number of less important 
 interruptions. But although these breaks exist, 
 we find pervading the works of many geologists a 
 belief, resulting from the evidence presented to 
 their minds, sometimes avowed, sometimes uncon- 
 sciously implied, that the succession of species 
 bears some definite relation to the succession of 
 strata. Thus Professor Ramsay says, that ' In 
 cases of superposition of fossiliferous strata, in pro- 
 portion as the species are more or less continuous, 
 that is to say, as the break in the succession of life 
 is partial or complete, so was the time that elapsed 
 between the close of the lower and the commence- 
 ment of the upper strata a shorter or a longer inter- 
 val. The break in life may be indicated not only by 
 a difference in species, but yet more importantly by
 
 314 CONTINUITY. 
 
 the absence of older and appearance of newer 
 allied or unallied genera.' 
 
 Indications of the connection between cosmical 
 studies and geological researches are dawning on 
 us : there is, for instance, some reason to believe 
 that we can trace many geological phenomena to 
 our varying rotation round the sun; thus more 
 than thirty years ago Sir J. Herschel proposed an 
 explanation of the changes of climate on the earth's 
 surface as evidenced by geological phenomena, 
 founded on the changes of eccentricity in the 
 earth's orbit. 
 
 He said he had entered on the subject * im- 
 pressed with the magnificence of that view of 
 geological revolutions which regards them rather 
 as regular and necessary efforts of great and 
 general causes, than as resulting from a series of 
 convulsions and catastrophes regulated by no laws 
 and reducible to no fixed principles.' 
 
 As the mean distance of the earth from the sun 
 is nearly invariable, it would seem at first sight 
 that the mean annual supply of light and heat 
 received by the earth would also be invariable; 
 but according to his calculations it is inversely 
 proportional to the minor axis of the orbit: tliis 
 would give less heat when the eccentricity of the 
 earth's orbit is approaching towards or at its 
 minimum. Mr. Croll has recently shown reason 
 to believe that the climate, at all events in the
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 315 
 
 circumpolar and temperate zones of the earth, 
 would depend on whether the winter of a given 
 region occurred when the earth at its period of 
 greatest eccentricity was in aphelion or perihelion 
 if the former, the annual average of temperature 
 would be lower, if the latter, it would be higher 
 than when the eccentricity of the earth's orbit 
 were less or approached more nearly to a circle. 
 He calculates the difference in the amount of heat 
 at the period of maximum eccentricity of the 
 earth's orbit to be as 19 to 26, according as the 
 winter would take place when the earth was in 
 aphelion or in perihelion. His reason may be 
 briefly stated thus : assuming the mean annual heat 
 to be the same, whatever the eccentricity of orbit, 
 yet if the extremes of heat and cold in winter and 
 summer be greater, a colder climate will prevail, 
 for there will be more snow and ice accumulated 
 in the cold winter than the hot summer can melt, 
 a result aided by the shelter from the sun's rays 
 produced by the vapour suspended in consequence 
 of the aqueous evaporation; hence we should get 
 glacial periods, when the orbit of the earth is at its 
 greatest eccentricity, at those parts of the earth's 
 surface where it is winter when the earth is in 
 aphelion; carboniferous or hot periods where it is 
 winter in perihelion; and normal or temperate 
 periods when the eccentricity of orbit is at a 
 minimum; all these would gradually slide into
 
 31 6 CONTINUITY. 
 
 each other, and would produce at long distant 
 periods alternations of cold and heat, several of 
 which we actually observe in geological records. 
 
 If this theory be borne out, we should approxi- 
 mate to a test of the time which has elapsed 
 between different geological epochs. Mr. Croll's 
 computation of this would make it certainly not 
 less than 100,000 years since the last glacial epoch, 
 a time not very long in geological clironology pro- 
 bably it is much more. 
 
 When we compare with the old theories of the 
 earth, by which the apparent changes on its sur- 
 face were accounted for by convulsions and cata- 
 clysms, the modern view inaugurated by Lyell, 
 your former President, and now, if not wholly, at 
 all events to a great extent adopted, it seems 
 strange that the referring past changes to similar 
 causes to those which are now in operation should 
 have remained uninvestigated until the present 
 century ; but with this, as with other branches of 
 knowledge, the most simple is frequently the latest 
 view which occurs to the mind. It is much more 
 easy to invent a Deux ex machind than to trace 
 out the influence of slow continuous change ; the 
 love of the marvellous is so much more attractive 
 than the patient investigation of truth, that we 
 find it to have prevailed almost universally in the 
 early stages of science. 
 
 In astronomy we had crystal spheres, cycles,
 
 ADDEESS TO THE EEITISH ASSOCIATION, 1866. 317 
 
 and epicycles; in chemistry the philosopher's 
 stone, the elixir vitas, the archseus or stomach 
 demon, and phlogiston; in electricity the notion 
 that amber possessed a soul, and that a mysterious 
 fluid could knock down a steeple. In geology a 
 deluge or a volcano was supplied. In palaeontology 
 a new race was created whenever theory required 
 it : how such new races began, the theorist did not 
 stop to enquire. 
 
 A curious speculator might say to a palaeontolo- 
 gist of even recent date, in the words of Lucretius, 
 
 ' Nam neque de coelo cecidisse animalia possunt 
 Nee terrestria de salsis exisse lacunis. 
 
 * # # * # 
 
 -E nihilo si crescere possent, 
 
 (Turn) fierent juvenes subito ex infantibus parvis, 
 E terraque exorta repente arbiista salirent ; 
 Quorum nil fieri manifestum est, omnia quando 
 Paulatim crescunt, ut par est, semine certo, 
 Crescentesque genus servant ' 
 
 which may be thus freely paraphrased : ' You 
 have abandoned the belief in one primaeval creation 
 at one point of time, you cannot assert that an ele- 
 phant existed when the first saurians roamed over 
 earth and water. Without, then, in any way 
 limiting Almighty power, if an elephant were 
 created without progenitors, the first elephant must, 
 in some way or other, have physically arrived on 
 this earth. Whence did he come? did he fall from
 
 318 CONTINUITY. 
 
 the sky (i.e. from interplanetary space)? did he 
 rise moulded out of a mass of amorphous earth or 
 rock? did he appear out of the cleft of a tree? 
 If he had no antecedent progenitors, some such 
 beginning must be assigned to him.' I know of 
 no scientific writer who has, since the discoveries 
 of geology have become familiar, ventured to pre- 
 sent in intelligible terms any definite notion of 
 how such an event could have occurred : those who 
 do not adopt some view of continuity are content to 
 say God willed it ; but would it not be more reve- 
 rent and more philosophical to enquire by observa- 
 tion and experiment, and to reason from induction 
 and analogy, as to the probabilities of such fre- 
 quent miraculous interventions? 
 
 I know I am touching on delicate ground, and 
 that a long time may elapse before that calm en- 
 quiry after truth which it is the object of associa- 
 tions like this to promote can be fully attained ; but 
 I trust that the members of this body are suffi- 
 ciently free from prejudice, whatever their opinions 
 may be, to admit an enquiry into the general 
 question whether what we term species are and 
 have been rigidly limited, and have at numerous 
 periods been created complete and unchangeable, 
 or whether, in some mode or other, they have not 
 gradually and indefinitely varied, and whether the 
 changes due to the influence of surrounding circum- 
 stances, to efforts to accommodate themselves to
 
 ADDKESS TO THE BKITISH ASSOCIATION, 1866. 319 
 
 surrounding changes, to what is called natural 
 selection, or to the necessity of yielding to superior 
 force in the struggle for existence, as maintained by 
 our illustrious countryman Darwin, and other causes, 
 have not so modified organisms as to enable them to 
 exist under changed conditions. I am not going to 
 put forward any theory of my own, I am not going to 
 argue in support of any special theory, but having 
 endeavoured to show how, as science advances, the 
 continuity of natural phenomena becomes more 
 apparent, it would be cowardice not to present 
 some of the main arguments for and against con- 
 tinuity as applied to the history of organic beings. 
 As we detect no such phenomenon as the crea- 
 tion or spontaneous generation of vegetables and 
 animals which are large enough for the eye to see 
 without instrumental assistance, as we have long 
 ceased to expect to find a Plesiosaurus spontane- 
 ously generated in our fish-pond, or a Pterodactyle 
 in our pheasant-cover, the field of this class of re- 
 search has become identified with the field of the 
 microscope, and at each new phase the investigation 
 has passed from a larger to a smaller class of or- 
 ganisms. The question whether among the small- 
 est and apparently the most elementary forms of 
 organic life the phenomenon of spontaneous gene- 
 ration obtains, has recently formed the subject of 
 careful experiment and animated discussion in 
 France. If it could be found that organisms of a
 
 320 CONTINUITY. 
 
 complex character were generated without progeni- 
 tors out of amorphous matter, it might reasonably 
 be argued that a similar mode of creation might 
 obtain in regard to larger organisms. Although 
 we see no such phenomenon as the formation of an 
 animal such as an elephant, or a tree such as an 
 oak, excepting from a parent which resembles it, yet 
 if the microscope revealed to us organisms, smaller 
 but equally complex, so formed without having 
 been reproduced, it would render it not improbable 
 that such might have been the case with larger 
 organic beings. The controversy between M. 
 Pasteur and M. Pouchet has led to a very close in- 
 vestigation of this subject, and the general opinion 
 is that when such precautions are taken as exclude 
 from the substance submitted to experiment all 
 possibility of germs from the atmosphere being in- 
 troduced, as by passing the air which is to support 
 the life of the animalcule through tubes heated to 
 redness and other precautions, no formation of or- 
 ganisms takes place. Some experiments of Mr. 
 Child's, communicated to the Royal Society during 
 the last year, again throw doubt on the negative 
 results obtained by M. Pasteur; so that the ques- 
 tion maybe not finally determined, but the balance 
 of experiment and opinion is against spontaneous 
 generation. 
 
 One argument presented by M. Pasteur is well 
 worthy of remark, viz. that in proportion as our
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 321 
 
 means of scrutiny become more searching, hetero- 
 geny, or the developement of organisms without 
 generation from parents of similar organism, has 
 been gradually driven from higher to lower forms 
 of life, so that if some apparent exceptions still 
 exist they are of the lowest and simplest forms, 
 and these exceptions may probably be removed, as 
 M. Pasteur considers he has removed them, by a 
 more searching investigation. 
 
 If it be otherwise, if heterogeny obtains at all, 
 few will not now admit that at present the result 
 of the most careful experiments shows it to 
 be confined to the more simple organic struc- 
 tures, and that all the progressive and more 
 highly developed forms are, as far as the most 
 enlarged experience shows, generated by repro- 
 duction. 
 
 The great difficulty which is met with at the 
 threshold of enquiry into the origin of species is 
 the definition of species ; in fact, species can hardly 
 be defined without begging the question in dis- 
 pute. 
 
 Thus if species be said to be a perseverance of 
 type incapable of blending itself with other types, 
 or, which comes nearly to the same thing, incapable 
 of producing by union with other types offspring 
 of an intermediate character which can again re- 
 produce, we arrive at this result, that whenever 
 the advocate of continuity shows a blending of
 
 322 CONTINUITY. 
 
 what had been hitherto deemed separate species, 
 the answer is, they were considered separate 
 species by mistake, they do not now come under 
 the definition of species, because they interbreed. 
 
 The line of demarcation is thus ex hypothesi re- 
 moved a step further, so that unless the advocate 
 of continuity can, on his side, prove the whole 
 question in dispute, by showing that all can directly 
 or by intermediate varieties reproduce, he is de- 
 feated by the definition itself of species. 
 
 On the other hand, if this, or something in fact 
 amounting to it, be not the definition of species 
 if it be admitted that distinct species can, under 
 certain favourable conditions, produce intermediate 
 offspring capable of reproduction, then continuity 
 in some mode or other is admitted. 
 
 The question then takes this form. Are there 
 species or are there not? Is the word to be used 
 as signifying a real, natural distinction, or as a 
 mere convenient designation applied to subdivi- 
 sions having a permanence which will probably 
 outlive man's discussions on the subject, but not 
 an absolute fixity ? The same question, in a wider 
 sense, and taking into consideration a much longer 
 time, would be applicable to genera and families. 
 
 Actual experiment has done little to elucidate 
 the question, nor, unless we can suppose the expe- 
 riments continued through countless generations, 
 is it likely to contribute much to its solution. We
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 323 
 
 must therefore have recourse to the enlarged ex- 
 perience or induction from the facts of geology, 
 paleontology, and physiology, aided by analogy 
 from the laws of action which nature evidences in 
 other departments. 
 
 The doctrine of gradual succession is hardly yet 
 formularised, and though there are some high 
 authorities for certain modifications of such view, 
 the preponderance of authority would necessarily 
 be on the other side. Geology and palaeontology 
 are recent sciences, and we cannot tell what the 
 older authors would have thought or written had 
 the more recently discovered facts been presented 
 to their view. Authority, therefore, does not much 
 help us on this -question. 
 
 Geological discoveries seemed, in the early period 
 of the science, to show complete extinction of 
 certain species and the appearance of new ones, 
 great gaps existing between the characteristics of 
 the extinct and the new species. As science ad- 
 vanced, these were more or less filled up, and the 
 difficulty in the first instance of admitting unlimited 
 modification of species would seem to have arisen 
 from the comparison of the extreme ends of the 
 scale where the intermediate links or some of 
 them were wanting. 
 
 To suppose a Zoophyte the progenitor of a 
 Mammal, or to suppose at some particular period 
 of time a highly developed animal to have come
 
 324 CONTINUITY. 
 
 out of nothing, or suddenly grown out of inorganic 
 matter, would appear at first sight equally extra va 
 gant hypotheses. As an effort of Almighty crea- 
 tive power, neither of these alternatives presents 
 more difficulty than the other ; but as we have no 
 means of ascertaining how creative power worked, 
 but by an examination and study of the works 
 themselves, we are not likely to get either virw 
 proved to ocular demonstration. A single pli;isi> 
 in the progress of natural transmutation would 
 probably require a term far transcending all that 
 embraced by historical records; and on the other 
 hand, it might be said, sudden creations, though 
 taking place frequently, if viewed with reference to 
 the immensity of time involved in geolo^icil 
 periods, may be so rare with reference to our ex- 
 perience, and so difficult of clear authentication, 
 that the non-observation of such instances cannot 
 be regarded as absolute disproof of their possible 
 occurrence. 
 
 The more the gaps between species are filled up 
 by the discovery of intermediate varieties the 
 stronger becomes the argument for transmutation 
 and the weaker that for successive creations, be- 
 cause the former view then becomes more and 
 more consistent with experience, the latter more 
 discordant from it. As undoubted cases of varia- 
 tion, more or less permanent, from given characn r- 
 istics, are produced by the effects of climate, food,
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 325 
 
 domestication, &c., the more species are increased 
 by intercalation, the more the distinctions slide 
 down towards those which are within the limits of 
 such observed deviations ; while on the other hand, 
 to suppose the more and more frequent recurrence 
 of fresh creations out of amorphous matter is a 
 multiplication of miracles or special interventions 
 not hi accordance with what we see of the uniform 
 and gradual progress of .nature, either in the or- 
 ganic or inorganic world. If we were entitled to 
 conclude that the progress of discovery would con- 
 tinue in the same course, and that species would 
 become indefinitely multiplied, the distinctions 
 would become infinitely minute, and all lines of 
 demarcation would cease, the polygon would be- 
 come a circle, the succession of points a line. 
 Certain it is that the more we observe the more 
 we increase the subdivision of species, and con- 
 sequently the number of these supposed creations ; 
 so that new creations become innumerable, and yet 
 of these we have no one well-authenticated instance, 
 and in no other observed operation of nature have 
 we seen this want of continuity, these frequent per 
 saltum deviations from uniformity, each of which 
 is a miracle. 
 
 The difficulty of producing intermediate offspring 
 from what are termed distinct species and the iii- 
 fecundity in many instances of hybrids are used as 
 strong arguments against continuity of succession ;
 
 326 CONTINUITY. 
 
 on the other hand, it may be said long-continued 
 variation through countless generations has given 
 rise to such differences of physical character that 
 reproduction is difficult in some cases, and in others 
 impossible. 
 
 Suppose, for instance, M to represent a parent- 
 race whose offspring by successive changes through 
 eons of time have divaricated, and produced on the 
 one hand a species A, and on the other a species Z, 
 the changes here have been so great that we should 
 never expect directly to reproduce an intermediate 
 between A and Z. A and B on the one hand, 
 and Y and Z on the other, might reproduce; but 
 to regain the original type M, we must not only 
 retrocede* tlirough all the intermediates, but must 
 have similar circumstances recalled in an inverse 
 order at each phase of retrogression, conditions 
 which it is obviously impossible to fulfil. But 
 though among the higher forms of organic struc- 
 ture we cannot retrace the effects of time and 
 reproduce intermediate types, yet among some of 
 the lower forms we find it difficult to assign any 
 line of specific demarcation ; thus as a result of the 
 very elaborate and careful investigations of Dr. 
 Carpenter on Foraminifera, he states, ' It has been 
 shown that a very wide range of variation exists 
 among Orbitolites, not merely as regards external 
 form, but also as to plan of developement ; arid not 
 merely as to the shape and aspect of the entire
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 327 
 
 organism, but also with respect to the size and 
 configuration of its component parts. It would 
 have been easy, by selecting only the most diver- 
 gent types from amongst the whole series of speci- 
 mens which I have examined, to prefer an appa- 
 rently substantial claim on behalf of these to be 
 accounted as so many distinct species. But after 
 having classified the specimens which could be 
 arranged around these types, a large proportion 
 would yet have remained, either presenting charac- 
 ters intermediate between those of two or more of 
 them, or actually combining those characters in 
 different parts of their fabric ; thus showing that 
 no lines of demarcation can be drawn across any 
 part of the series that shall definitely separate it 
 into any number of groups, each characterised by 
 features entirely peculiar to itself.' 
 
 At the conclusion of his enquiry he states 
 
 I. The range of variation is so great among 
 Foraminifera as to include not merely the differen- 
 tial characters which systematists proceeding upon 
 the ordinary methods have accounted specific, but 
 also those upon which the greater part of the 
 genera of this group have been founded, and even 
 in some instances those of its orders. 
 
 II. The ordinary notion of species as assemblages 
 of individuals marked out from each other by defi- 
 nite characters that have been genetically trans- 
 mitted from original proto-types similarly distin-
 
 328 CONTINUITY. 
 
 guished, is quite inapplicable to this group ; since 
 even if the limits of such assemblages were ex- 
 tended so as to include what elsewhere would be 
 accounted genera, they would still be found so 
 intimately connected by gradational links that 
 definite lines could not be drawn between them. 
 
 III. The only natural classification of the vast 
 aggregate of diversified forms which tin's group 
 contains will be one which ranges them according 
 to their direction and degree of divergence from a 
 small number of principal family types ; and any 
 subordinate grouping of genera and species which 
 may be adopted for the convenience of description 
 and nomenclature must be regarded merely as 
 assemblages of forms characterised by the nature 
 and degree of the modifications of the original type, 
 which they may have respectively acquired in the 
 course of genetic descent from a common ancestry. 
 
 IV. Even in regard to these family types it may 
 fairly be questioned whether analogical evidence 
 does not rather favour the idea of their derivation 
 from a common original than that of their primi- 
 tive distinctness. 
 
 Mr. H. Bates, when investigating ' The Lepidop- 
 tera of the Amazon Valley,' may almost be said to 
 have witnessed the origin of some species of Butter- 
 flies, so close have been his observations on the 
 habits of these animals that have led to their varia- 
 tion and segregation, so closely do the results
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 329 
 
 follow his observations, and so great is the diffi- 
 culty of otherwise accounting for any of the observed 
 facts. 
 
 In the numerous localities of the Amazon region 
 certain gregarious species of Butterfly (Heliconidea) 
 swarm in incredible numbers, almost outnumbering 
 all the other butterflies in the neighbourhood ; the 
 species in the different localities being different, 
 though often to be distinguished by a very slight 
 shade. 
 
 In these swarms are to be found, in small num- 
 bers, other species of butterflies belonging to as 
 many as ten different genera, and even some moths ; 
 and these intruders, though they structurally differ 
 in toto from the swarms they mingle with, and 
 from one another, mimic the Heliconidece so closely 
 in colours, habits, mode of flight, &c., that it is 
 almost impossible to distinguish the intruders from 
 those they mingle with. The obvious benefit of 
 this mimicry is safety, the intruders hence escaping 
 detection by predatory animals. 
 
 Mr. Bates has extended his observations to the 
 habits of life, food, variations and geographical 
 range of the species concerned in these mimetic 
 phenomena, and finds in every case corroborative 
 evidence of every variety and species being deriva- 
 tive, the species being modified from place to place 
 to suit the peculiar form of Heliconidea stationed 
 there.
 
 33 CONTINUITY. 
 
 Mr. Wallace has done similar service to the deri- 
 vative theoiy by his observations and writings on 
 the Butterflies and Birds of the Malay Archipelago, 
 adducing instances of mimetic resemblances strictly 
 analogous to the above; and adding in further 
 illustration a beautiful series of instances where 
 the form of the wing of the same butterfly is so 
 modified in various islets as to produce changes in 
 their mode of flight that tend to the conservation 
 of the variety by aiding its escape when chased by 
 birds or predacious insects. 
 
 He has also adduced a multitude of examples of 
 geographical and representative species, races, and 
 varieties, forming so graduated a series as to 
 render it obvious that they have had a common 
 origin. 
 
 The effect of food in the formation and segrega- 
 tion of races and of certain groups of insects has 
 been admirably demonstrated by Mr. B. D. Walsh, 
 of North America. 
 
 Mr. McDonnell has been led to the discovery of 
 a new organ in electric fishes from the application 
 of the theory of descent, and Dr. Fritz Miiller has 
 published numerous observations showing that or- 
 gans of very different structure may, through the 
 operation of natural selection, acquire very similar 
 and even identical functions. Sir John Lubbock's 
 diving hymenopterous insect affords a remarkable 
 illustration of analogous phenomena; it dives by
 
 ADDKESS TO THE BRITISH ASSOCIATION, 1S66. 331 
 
 the aid of its wings, and is the only insect of the 
 vast order it belongs to that is at all aquatic. 
 
 The discovery of the Eozoon is of the highest 
 importance in reference to the derivative hypothe- 
 sis, occurring as it does in strata that were formed 
 at a period inconceivably antecedent to the presup- 
 posed introduction of life upon the globe, and dis- 
 placing the argument derived from the supposition 
 that at the dawn of life a multitude of beings of 
 high organisation were simultaneously developed 
 (in the Silurian and Cambrian strata). 
 
 Professor A. De Candolle, one of the most dis- 
 tinguished continental botanists, has, to some 
 extent, abandoned the tenets held in his ' Geogra- 
 phic Botanique,' and favours the derivative hypo- 
 thesis in his paper on the variation of oaks; 
 following up a paper, by Dr. Hooker, on the oaks 
 of Palestine, showing that some sixteen of them 
 are derivative, he avows his belief that two-thirds 
 of the 300 species of this genus, which he himself 
 describes, are provisional only. 
 
 Dr. Hooker, who had only partially accepted the 
 derivative hypothesis propounded before the publi- 
 cation of ' The Origin of Species through Natural 
 Selection,' at the same time declining the doctrine 
 of special creation, has since then cordially adopted 
 the former, and illustrated its principles by apply- 
 ing them to the solution of various botanical ques- 
 tions : first, in reference to the flora of Australia, 

 
 332 CONTINUITY. 
 
 the anomalies of which he appears to explain 
 satisfactorily by the application of these principles ; 
 and, latterly, in reference to the Arctic flora. 
 In the case of the Arctic flora, he believes that 
 originally Scandinavian types were spread over the 
 high northern latitudes ; that these were driven 
 southwards during the glacial period, when many 
 of them changed their forms hi the struggle that 
 ensued with the displaced temperate plants; that 
 on the returning warmth, the Scandinavian plants, 
 whether changed or not, were driven again north- 
 wards and up to the mountains of the temperate 
 latitudes, followed in both cases by series of pre- 
 existing plants of the temperate Alps. The result 
 is the present mixed Arctic flora, consisting of a 
 basis of more or less changed and unchanged Scan- 
 dinavian plants, associated in each longitude with 
 representatives of the mountain flora of the more 
 temperate regions to the south of them. 
 
 The publication of a previously totally unknown 
 flora, that of the Alps of tropical Africa, by Dr. 
 Hooker, has afforded a multitude of facts that have 
 been applied iti confirmation of the derivative 
 hypothesis. This flora is found to have relation- 
 ships with those of temperate Europe and North 
 Africa, of the Cape of Good Hope, and of the 
 mountains of tropical Madagascar and Abyssinia, 
 that can be accounted for on no other hypothesis 
 but that there has been ancient climatal connexion
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 333 
 
 and some coincident or subsequent slight changes 
 of specific character. 
 
 The doctrine of Cuvier, every day more and 
 more borne out by observation, that each organ 
 bears a definite relation to the whole of the indi- 
 vidual, seems to support the view of indefinite 
 variation. If an animal seeks its food or safety by 
 climbing trees, its claws will become more pre- 
 hensile, the muscles which act upon those claws 
 must become more developed, the body will be- 
 come agile by the very exercise which is necessary 
 to it, and each portion of the frame will mould 
 itself to the wants of the animal by the effect on it 
 of the habits of the animal. 
 
 Another series of facts which present an argu- 
 ment in favour of gradual succession are the 
 phases of resemblance to inferior orders which the 
 embryo passes through in its developenaent, and the 
 relations shown in what is termed the metamor- 
 phosis of plants; facts difficult to account for on 
 the theory of frequent separate creations, but al- 
 most inevitable on that of gradual succession. So 
 also, the existence of rudimentary and effete organs, 
 which must either be referred to a lusus natures or 
 to some mode of continuous succession. 
 
 The doctrine of typical nuclei seems only a 
 mode of evading the difficulty ; experience does 
 not give us the types of theory, and, after all, 
 what are these types ? It must be admitted there
 
 33 4 CONTINUITY. 
 
 are none such in reality ; how are we led to the 
 theory of them? simply by a process of abstraction 
 from classified existences, Having grouped from 
 natural similitudes certain forms into a class, we 
 select attributes common to each member of the 
 class, and call the assemblage of such attributes a 
 type of the class. This process gives us an ab- 
 stract idea, and we then transfer this idea to the 
 Creator, and make Him start with that which our 
 own imperfect generalisation has derived. It 
 seems to me that the doctrine of types is, in fact, 
 a concession to the theory of continuity or inde- 
 finite variability; for the admission that large 
 groups have common characters shows, necessarily, 
 a blending of forms within the scope of the group, 
 which supports the view of each member bcii in- 
 derived from some other member of it : can it be 
 asserted that the assigned limits of such groups 
 have a definite line of demarcation ? 
 
 The condition of the earth's surface, or at least 
 of large portions of it, has for long periods re- 
 mained substantially the same ; this would involve 
 a greater degree of fixity in the organisms which 
 have existed during such periods of little change 
 than in those which have come into being during 
 periods of more rapid transition; for, though 
 rejecting catastrophes as the general modus agendi 
 of nature, I am far from saying that the march of 
 physical changes has been always perfectly uniform.
 
 ADDEESS TO THE BRITISH ASSOCIATION, 1866. 335 
 
 There have been doubtless what may be termed 
 secular seasons, and there have been local changes 
 of varying degrees of extent and permanence; 
 from such causes organised beings would be more 
 concentrated in certain directions than in others, 
 the fixity of character being in the ratio of the fixity 
 of condition. This would throw natural forms 
 into certain gr-oups which would be more prominent 
 than others, like the colours of the rainbow, which 
 present certain predominant tints though they 
 merge into each other by insensible gradations. 
 
 While the evidence seems daily becoming stronger 
 in favour of a derivative hypothesis as applied to 
 the succession of organic beings, we are far re- 
 moved from anything like a sufficient number of 
 facts to show that, at all events within the ex- 
 isting geological periods capable of being inves- 
 tigated, there has been any great progression from 
 a simpler or more embryonic to a more complex 
 type. 
 
 Professor Huxley, though inclined to the de- 
 rivative hypothesis, shows, in the concluding por- 
 tion of his address to the Geological Society, 1862, 
 a great number of cases in which, though there is 
 abundant evidence of variation, there is none of 
 progression. There are, however, several groups 
 of Yertebrata in which the endoskeleton of the 
 older presents a less ossified condition than that 
 of the younger genera. He cites the Devonian
 
 336 CONTINUITY. 
 
 Ganoids, the Mesozoic Lepidosteidae, the Palaeozoic 
 Sharks, and the more ancient Crocodilia and Lacer.- 
 tilia, and particularly the Pycnodonts and Laby- 
 rinthodonts, as instances of this when compared 
 with their more recent representatives. 
 
 The records of life on the globe may have been 
 destroyed by the fusion of the rocks, which would 
 otherwise have preserved them, or by crystallisa- 
 tion after hydrothermal action. The earlier forms 
 may have existed at a period when this planet was 
 in course of formation, or being segregated or de? 
 tached from other worlds or systems. We have 
 not evidence enough to speculate on the subject, 
 but by time and patience we may acquire it. 
 
 Were all the forms which have existed en> 
 balmed in rock the question would be solved; but 
 what a small proportion of extinct forms is so 
 preserved, and must be, if we consider the circum- 
 stances necessary to fossilise organic remains. On 
 the dry land, unwashed by rivers and seas, when 
 an animal or plant dies, it undergoes chemical de- 
 composition which changes its form ; it is consumed 
 by insects, its skeleton is oxidised and crumbles 
 into dust. Of the myriads of animals and vege- 
 tables which annually perish we find hardly an 
 instance of a relic so preserved as to be likely to 
 become a permanent fossil. So again in the deeper 
 parts of the ocean, or of the larger lakes, the lou- 
 tish there are perish and their remains sink to the
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 337 
 
 bottom, and are there frequently consumed by 
 other marine or lacustrine organisms or chemically 
 decomposed. As a general rule, it is only when 
 the remains are silted up by marine, fluviatile or 
 lacustrine sediments that the remains are preserved. 
 Geology, therefore, might be expected to keep for 
 us mainly such organic remains as inhabited deltas 
 or the margins of seas, lakes, or rivers ; here and 
 there an exception may occur, but the mass of pre- 
 served relics would be those of creatures so si- 
 tuated ; and so we find it, the bulk of fossil remains 
 consists of fish and amphibia, shell-fish form the 
 major part of the geological museum, limestone 
 and chalk rocks frequently consisting of little else 
 than a congeries of fossil shells. Plants of reed or 
 rush-like character, fish which are capable of in- 
 habiting shallow waters, and saurian animals, form 
 another large portion of geological remains. 
 
 Compare the shell-fish and amphibia of existing 
 organisms with the other forms, and what a small 
 proportion they supply ; compare the shell-fish and 
 amphibia of Palaeontology with the other forms, 
 and what an overwhelming majority they yield. 
 
 There is nothing, as Professor Huxley has re- 
 marked, like an extinct order of birds or mammals, 
 only a few isolated instances. It may be said the 
 ancient world possessed a larger proportion of fish 
 and amphibia, and was more suited to their 
 existence. I see no reason for believing this, at 
 z
 
 S3S CONTINUITY. 
 
 least to anything like the extent contended for; 
 the fauna and flora now in course of being pre- 
 served for future ages would give the same idea to 
 our successors. 
 
 Crowded as Europe is with cattle, birds, in- 
 sects, &c., how few are geologically preserved ! 
 while the muddy or sandy margins of the ocean, 
 the estuaries, and deltas are yearly accumulat- 
 ing numerous Crustacea and mollusca, with some 
 fishes and reptiles, for the study of future palaeon- 
 tologists. 
 
 If this position be right, then, notwithstanding 
 the immense number of preserved fossils, there 
 must have lived an immeasurably larger number 
 of unpreserved organic beings, so that the chance 
 of filling up the missing links, except in occasional 
 instances, is very slight. Yet where circumstances 
 have remained suitable for their preservation, many 
 closely connected species are preserved in other 
 words, while the intermediate types in certain cases 
 are lost, in others they exist. The opponents of 
 continuity lay all stress on the lost and none on 
 the existing links. 
 
 But there is another difficulty in the way of 
 tracing a given organism to its parent forms, which, 
 from our conventional mode of deducing genealogies, 
 is never looked upon in its proper light. 
 
 Where are we to look for the remote ancestor of 
 a given form ? Each of us, supposing none of our
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 339 
 
 progenitors to have intermarried with relatives, 
 would have had at or about the period of the Nor- 
 man Conquest upwards of a hundred million direct 
 ancestors of that generation, and if we add the in- 
 termediate ancestors, double that number. As 
 each individual has a male and female parent, we 
 have only to multiply by two for each thirty years, 
 the average duration of a generation, and it will 
 give the above result. 
 
 Let any one assume that one of his ancestors at 
 the time of the Norman Conquest was a Moor, 
 another a Celt, and a third a Laplander, and that 
 these three were preserved while all the others 
 were lost, he would never recognise either of them 
 as his ancestor, he would only have the one-hundred 
 millionth of the blood of each of them, and as far 
 as they were concerned there would be no per- 
 ceptible sign of identity of race. 
 
 But the problem is more complex than that 
 which I have stated; at the time of the Conquest 
 there were hardly a hundred million people in 
 Europe, it follows that a great number of the an- 
 cestors of the propositus must have intermarried 
 with relations, and then the pedigree, going back 
 to the time of the Conquest, instead of being repre- 
 sented by diverging lines, would form a network 
 so tangled that no skill could unravel it; the law 
 of probabilities would indicate that any two people 
 in the same country, taken at hazard, would not 
 
 x3
 
 340 CONTINUITY. 
 
 have many generations to go back before they would 
 find a common ancestor, who probably, could they 
 have seen him or her in the life, had no traceable 
 resemblance to either of them. Thus two animals 
 of a very different form, and of what would be 
 termed very different species, might have a common 
 geological ancestor, and yet the skill of no compa- 
 rative anatomist could trace the descent. 
 
 From the long-continued conventional habit of 
 tracing pedigrees through the male ancestor, we 
 forget in talking of progenitors that each individual 
 has a mother as well as a father, and there is no 
 reason to suppose that he has in him less of the 
 blood of the one than of the other. 
 
 The recent discoveries in palaeontology show us 
 that Man existed on this planet at an epoch far an- 
 terior to that commonly assigned to him. The 
 instruments connected with human remains, and 
 indisputably the work of human hands, show that 
 to these remote periods the term civilisation could 
 hardly be applied chipped flints of the rudest 
 construction, probably, in the earlier cases, fabri- 
 cated by holding an amorphous flint in the hand 
 and chipping off portions of it by striking it 
 against a larger stone or rock ; then, as time sug- 
 gested improvements, it would be more carefully 
 shaped, and another stone used as a tool; then (at 
 what interval we can hardly guess) it would be 
 ground, then roughly polished, and so on subse-
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 341 
 
 quently bronze weapons, and nearly the last before 
 we come to historical periods, iron. Such an appa- 
 rently simple invention as a wheel must, in all 
 probability, have been far subsequent to the rude 
 hunting-tools or weapons of war to which I have 
 alluded. 
 
 A little step-by-step reasoning will convince the 
 unprejudiced that what we call civilisation must 
 have been a gradual process; can it be supposed 
 that the inhabitants of Central America or of 
 Egypt suddenly and what is called instinctively 
 built their cities, carved and ornamented their 
 monuments? If not, if they must have learned to 
 construct such erections, did it not take time to 
 acquire such learning, to invent tools as occasion 
 required, contrivances to raise weights, rules or 
 laws by which men acted in concert to effect the 
 design ? Did not all this require time ? and if, as 
 the evidence of historical times shows, invention 
 marches with a geometrical progression, how slow 
 must have been the earlier steps! If even now 
 habit, and prejudice resulting therefrom, vested in- 
 terests, &c., retard for some time the general appli- 
 cation of a new invention, what must have been 
 the degree of retardation among the comparatively 
 uneducated beings which then existed ? 
 
 I have of course been able to indicate only a few 
 of the broad arguments on this most interesting 
 subject; for detailed results the works of Darwin,
 
 342 CONTINUITY. 
 
 Hooker, Huxley, Carpenter, Lyell, and others must 
 be examined. If I appear to lean to the view that 
 the successive changes in organic beings do not 
 take place by sudden leaps, it is, I believe, from no 
 want of an impartial feeling ; but if the facts are 
 stronger in favour of one theory than another, it 
 would be an affectation of impartiality to make the 
 balance appear equipoised. 
 
 The prejudices of education and associations with 
 the past are against this as against all new views ; 
 and while on the one hand a theory is not to be 
 accepted because it is new and primd facie plau- 
 sible, still to this assembly I need not say that its 
 running counter to existing opinions is not neces- 
 sarily a reason for its rejection ; the onus probandi 
 should rest on those who advance a new view, but 
 the degree of proof must differ with the nature of 
 the subject. The fair question is, Does the newly- 
 proposed view remove more difficulties, require 
 fewer assumptions, and present more consistency 
 with observed facts than that which it seeks to 
 supersede ? If so, the philosopher will adopt it, and 
 the world will follow the philosopher after many 
 days. 
 
 It must be borne in mind that even if we are 
 satisfied from a persevering and impartial enquiry 
 that organic forms have varied indefinitely in time, 
 the causa causans of these changes is not explained 
 by our researches ; if it be admitted that we find
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 343 
 
 no evidence of amorphous matter suddenly changed 
 into complex structure, still why matter should be 
 endowed with the plasticity by which it slowly 
 acquires modified structure is unexplained. If we 
 assume that natural selection, or the struggle for 
 existence, coupled with the tendency of like to 
 reproduce like, gives rise to various organic 
 changes, still our researches are at present unin- 
 structive as to why like should produce like, why 
 acquired characteristics in the parent should be 
 reproduced in the offspring. Reproduction itself 
 is still an enigma, and this great question may in- 
 volve deeper thoughts than it would be suitable to 
 enter upon now. 
 
 Perhaps the most convincing argument in favour 
 of continuity which could be presented to a doubt- 
 ing mind would be the difficulty it would feel in 
 representing to itself any per saltum act of nature. 
 Who would not be astonished at beholding an oak 
 tree spring up in a day, and not from seed or shoot ? 
 We are forced by experience, though often uncon- 
 sciously, to believe in continuity as to all effects 
 now taking place ; if any one of them be anomalous 
 we endeavour, by tracing its history and concomi- 
 tant circumstances, to find its cause, i.e. to relate 
 it to antecedent phenomena; are we then to reject 
 similar enquiries as to the past? is it laudable to 
 seek an explanation of present changes by observa- 
 tion, experiment, and analogy, and yet reprehen-
 
 344 CONTINUITY. 
 
 sible to apply the same mode of investigation to 
 the past history of the earth and of the organic 
 remains embalmed in it ? 
 
 If we disbelieve in sudden creations of matter 
 or force, in the sudden formations of complex 
 organisms now, if we now assign to the heat of the 
 sun an action enabling vegetables to live by assi- 
 milating gases and amorphous earths into growing 
 structures, why should such effects not have taken 
 place in earlier periods of the world's history, when 
 the sun shone as now, and when the same materials 
 existed for his rays to fall upon ? 
 
 If we are satisfied that continuity is a law of na- 
 ture, the true expression of the action of Almighty 
 Power, then, though we may humbly confess our 
 inability to explain why matter is impressed with 
 this tendency to gradual structural formation, we 
 should cease to look for special interventions of 
 creative power in changes which are difficult to 
 understand, because, being removed from us in 
 time, their concomitants are lost ; we should endea- 
 vour from the relics to evoke their history, and 
 when we find a gap not try to bridge it over with 
 a miracle. 
 
 If it be true that continuity pervades all physical 
 phenomena, the doctrine applied by Cuvier to the 
 relations of the different parts of an animal to each 
 other might be capable of great extension. All the 
 phenomena of inorganic and organised matter
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 345 
 
 might be expected to be so inter-related that the 
 study of an isolated phenomenon would lead to a 
 knowledge of numerous other phenomena with 
 which it is connected. As the antiquary deduces 
 from a monolith the tools, the arts, the habits, and 
 epoch of those by whom it is wrought, so the stu- 
 dent of science may deduce from a spark of elec- 
 tricity or ray of light the source whence it is 
 generated; and by similar processes of reasoning 
 other phenomena hitherto unknown may be de- 
 duced from their probable relation with the known. 
 But, as with heat, light, magnetism, and electricity, 
 though we may study the phenomena to which 
 these names have been given, and their mutual 
 relations, we know nothing of what they are ; so, 
 whether we adopt the view of natural selection, 
 of effort, of plasticity, &c., we know not why 
 organisms should have this nisus formativus, or 
 why the acquired habit or exceptional quality 
 of the individual should reappear in the off- 
 spring. 
 
 Philosophy ought to have no likes or dislikes, 
 truth is her only aim ; but if a glow of admiration 
 be permitted to a physical enquirer, to my mind a 
 far more exquisite sense of the beautiful is conveyed 
 by the orderly developement, by the necessary inter- 
 relation and inter-action of each element of the 
 cosmos, and by the conviction that a bullet falling 
 to the ground changes the dynamical conditions of
 
 346 CONTINUITY. 
 
 the universe, than can be conveyed by mysteries, 
 by convulsions, or by cataclysms. 
 
 The sense of understanding is to the educated 
 more gratifying than the love of the marvellous, 
 though the latter need never be wanting to the 
 nature -seeker. 
 
 But the doctrine of continuity is not solely ap- 
 plicable to physical enquiries. 
 
 The same modes of thought which lead us to see 
 continuity in the field of the microscope as in the 
 universe, in infinity downwards as in infinity up- 
 wards, will lead us to see it in the history of 
 our own race ; the revolutionary ideas of the so- 
 called natural rights of man, and a priori reason- 
 ing from what are termed first principles, are far 
 more unsound and give us far less ground for im- 
 provement of the race than the study of the 
 gradual progressive changes arising from changed 
 circumstances, changed wants, changed habits. 
 Our language, our social institutions, our laws, the 
 constitution of which we are proud, are the growth 
 of time, the product of slow adaptations, result- 
 ing from continuous struggles. Happily in this 
 country practical experience has taught us to im- 
 prove rather than to remodel ; we follow the law 
 of nature and avoid cataclysms. 
 
 The superiority of Man over other animals in- 
 habiting this planet, of civilised over savage man, 
 and of the more civilised over the less civilised, is
 
 ADDRESS TO THE BRITISH ASSOCIATION, 1866. 347 
 
 proportioned to the extent which his thought can 
 grasp of the past and of the future. His memory 
 reaches further back, his capability of prediction 
 reaches further forward in proportion as his know- 
 ledge increases. He has not only personal memory 
 which brings to his mind at will the events of his 
 individual life he has history, the memory of the 
 race ; he has geology, the history of the planet ; he 
 has astronomy, the geology of other worlds. 
 Whence does the conviction to which I have 
 alluded, that each material form bears in itself the 
 records of its past history, arise? Is it not from 
 the belief in continuity? Does not the worn 
 hollow on the rock record the action of the tide, 
 its stratified layers the slow deposition by which it 
 was formed, the organic remains imbedded in it 
 the beings living at the times these layers were 
 deposited, so that from a fragment of stone we can 
 get the history of a period myriads of years ago ? 
 From a fragment of bronze we may get the history 
 of our race at a period antecedent to tradition. 
 As science advances our power of reading such 
 history improves and is extended. Saturn's ring 
 may help us to a knowledge of how our solar 
 system developed itself, for it as surely contains 
 that history as the rock contains the record of its 
 own formation. 
 
 By this patient investigation how much have we 
 already learned, which the most civilised of ancient
 
 348 CONTINUITY. 
 
 human races ignored ! While in ethics, in politics, 
 in poetry, in sculpture, in painting, we have 
 scarcely, if at all, advanced beyond the highest 
 intellects of ancient Greece or Italy, how great are 
 the steps we have made in physical science and its 
 applications ! 
 
 But how much more may we not expect to 
 know? 
 
 We, this evening assembled, Ephemera as we 
 are, have learned by transmitted labour, to weigh, 
 as in a balance, other worlds larger and heavier 
 than our own, to know the length of their days 
 and years, to measure their enormous distance 
 from us and from each other, to detect and accu- 
 rately ascertain the influence they have on the 
 movements of our world and on each other, and to 
 discover the substances of which they are com- 
 posed ; may we not fairly hope that similar methods 
 of research to those which have taught us so much 
 may give our race further information, until pro- 
 blems relating not only to remote worlds, but 
 possibly to organic and sentient beings which may 
 inhabit them, problems which it might now seem 
 wildly visionary to enunciate, may be solved by 
 progressive improvements in the modes of apply- 
 ing observation and experiment, induction and 
 deduction.
 
 NOTES AND REFERENCES 
 TO THE CORRELATION OF PHYSICAL FORCES. 
 
 PAGE 
 
 7. THE reader who is curious as to the views of the ancients, 
 regarding the objects of science, will find clues to them 
 in the second book of ARISTOTLE'S Physics, and in the 
 first three books of the Metaphysics. See also the 
 Timjeus of PLATO, and RITTER'S History of Ancient 
 Philosophy, where a sketch of the Philosophy of 
 LEUCIPPUS and DEMOCRITUS will be found. 
 9. BACON'S Novum Organum, book ii. aph. 5 and 6. 
 11. HUME'S Enquiry concerning Human Understanding, S. 7, 
 London, 17-68. 
 
 BROWN'S Enquiry into the Relations of Cause and Effect, 
 London, 1835. 
 
 The illustration I have used of a floodgate has been ob- 
 jected to, as being one to which the term cause would 
 scarcely be applied, but after some consideration I have 
 retained it ; if cause be viewed only as sequence, it 
 must be limited to sequence under given conditions or 
 circumstances, and here, given the conditions, the se- 
 quence is invariable. I see no difference quoad the 
 argument, between this illustration and that of BROWN 
 of a lighted match and gunpowder (4th edit. p. 27), to 
 which my reasoning would equally well apply. 
 
 HERSCHEL'S Discourse on the Study of Natural Philosophy, 
 pp. 88 and 149. 
 
 13. Quarterly Review, vol. Ixviii. p. 212. 
 
 WHEWELL, On the Question ' Are Cause and Effect Succes- 
 sive or Simultaneous ? ' (Cambridge Philosophical 
 Transactions, vol. vii. p. 319).
 
 350 NOTES AND REFERENCES. 
 
 PAGE 
 
 14. HERSCHEL'S Discourse, p. 93. 
 
 AMPERE, Theorie des Pbenomenes Electro-dynamiques, 
 
 Memoirs in the Ann. de Chimie et de Physique, and 
 
 works from 1820 to 1826, Paris. 
 21. LAMARCK, ' Sur la Matiere du Son ' (Journal de Physique, 
 
 vol. xlix. p. 397). 
 24. D'ALEMBERT, Traite de Dynamique, pp. 3 and 4, Paris, 
 
 1796. 
 28. BABBAGE, On the Permanent Impression of our Words 
 
 and Actions on the Globe we inhabit, 9th Bridgewater 
 
 Treatise, ch. ix. 
 32. MAYER, Anualen der Pharmacie Leibig und Wohler, May 
 
 1852. 
 RANKING, Measure of Moving Force. (The Engineer, 
 
 October 26, 1866). 
 
 35. JOULE, On the Mechanical Equivalent of Heat (Phil. Trans. 
 
 1850, p. 61). 
 
 36. ERMAN, Influence of Friction upon Thermo-electricity 
 
 (Reports of the British Association, 1845). 
 
 39. BECQUEREL, D^gagement de I'Electricite" par Frottement, 
 
 Traite de 1'Electricite, torn. ii. p. 113 et seq. 
 
 40. SULLIVAN, Currents produced by the vibration of metals 
 
 (Archiv. de 1'Electricite, t. 10, p. 480). LEROUX, Vibra- 
 tions arrested produce heat (Cosmos, March 30, 1860). 
 
 42. WHEATSTONE, On the Prismatic Decomposition of Electrical 
 
 Light (Notices of Communications to the British Asso- 
 ciation, p. 11, 1835). 
 
 43. BACON, De Forma Calidi, Nov. Org. book 2, aph. 20. 
 RUMFORD, An Enquiry concerning the Source of Heat 
 
 which is excited by Friction (Phil. Trans, p. 80, 1798). 
 DAVY, On the Conversion of Ice into Water by Friction 
 
 (West of England Contributions, p. 16). 
 Of Heat or Calorific Repulsion (Elements of Chemical 
 
 Philosophy, p. 69). 
 46. BADEN POWELL, On the Repulsive Power of Heat (Phil. 
 
 Trans. 1834, p. 485). 
 FRESNEL, Annales de Chimie, torn. xxix. pp. 57 and 107.
 
 NOTES AND REFERENCES. 351 
 
 PAGE 
 
 47. MOSER, On Invisible Light (Taylor's Scientific Memoirs, 
 vol. iii. pp. 461 and 465). 
 
 49. BLACK, On Latent Heat (Elements of Chemistry, p. 144 
 et passim, 1803). 
 
 51. The experiments of HENRY and DONNY have shown that 
 the cohesion of liquids, as far as their antagonism to 
 rupture goes, is much greater than has been generally 
 believed. These experiments, however, make no dif- 
 ference in the view I have put forth, as, whatever be 
 the character of the attraction, there is a molecular 
 attraction to be overcome in changing bodies from the 
 solid to the liquid state, which must require and exhaust 
 force. 
 
 DONNY, Sur la Cohesion des Liquides (Memoires de 1' Aca- 
 demic Royale de Bruxelles, 1843). 
 
 HENRY, Proceedings of the American Philosophical Society, 
 April 1844 (Silliman's Journal, vol. xlviii. p. 215). 
 
 56. THILORIER, Solidification de 1'Acide carbonique (Ann. de 
 Ch. et de Phys. torn. Ix. p. 432). 
 
 59. I. WEDGWOOD, Thermometer for measuring the Higher 
 
 Degrees of Heat (Phil. Trans. 1782, p. 305 ; and 1786, 
 
 p. 390). 
 TYNDALL, On the Physical Properties of Ice (Phil. Trans. 
 
 1858, p. 211). 
 DESPRETZ, Kecherches sur le Maximum de Densite de 
 
 1'Eau pure et des Dissolutions aqueuses (Ann. de Ch. 
 
 et de Ph. torn. Ixx. p. 45, and torn. Ixxiii. p. 296). 
 
 60. BIOT (Comptes rendus de 1'Academie des Sciences, Paris 
 
 1850, p. 281). The experiments on circular polarisation 
 by water were, I believe, by Dr. Leeson. 
 
 62. I. THOMPSON, Trans. R. S. Edin. vol. xvi. p. 575. 
 W. THOMPSON, Phil. Mag. August 1850, p. 1 23. 
 
 BUNSEN, Pogg. Ann. vol. Ixxxi. p. 562 ; Ann. de Ch. et 
 de Phys. vol. xxxv. p. 383. Effects of Pressure on the 
 Freezing Point. 
 
 63. JOULE, Phil. Trans. 1852, p. 99. 
 
 Although, taking the phenomena as they are known
 
 352 NOTES. AND REFERENCES. 
 
 PAGE 
 
 to exist, the mechanical laws may be deduced, yet in 
 any physical conception of the nature of heat expansion 
 by cold has always been a great stumbling-block to me, 
 and I believe to many others. 
 
 63. DULONG and PETIT, and REGNAULT. See their Memoirs 
 abstracted and referred to in Gmelin's Handbook of 
 Chemistry, translated by Watts for the Cavendish 
 Society, vol. i. p. 242 et seq. 
 
 65. WOOD, Phil. Mag. 1851, 1852. 
 
 67. SEXARMONT, Conduction of Heat by Crystals (Gmelin's 
 
 Handbook, vol. i. p. 222). 
 
 68. KNOBLAUCH, Ann. de Ch. et de Ph. vol. xxxvi. p. 124. 
 TYNDALL, Transmission of Heat through Organic Struc- 
 tures (Phil. Trans, vol. cxliii. p. 217). 
 
 71. GROVE, Electricity produced by approximating Metals ; 
 
 Report of a Lecture at the London Institution (Literary 
 
 Gazette, 1843, p. 39). 
 GASSIOT, Phil. Mag. October 1844. 
 ROGET, On the Improbability of the Contact exciting 
 
 Force: Treatise on Galvanism (Library of Useful Know- 
 ledge, S. 113). 
 
 FARADAY, Phil. Trans. 1840, p. 126. 
 73. MELLONI, Sur la Polarisation de la Chaleur: Recherches 
 
 sur plusieurs Phenomenes calorifiques ( Annales de Chimie 
 
 et de Ph. torn. xlv. pp. 5-68; torn. xli. pp. 375-410; 
 
 torn, xlviii. pp. Ib8, 218). 
 FORBES, On the Refraction and Polarisation of Heat 
 
 (Transactions of the Royal Society of Edinburgh, vol. 
 
 xiii.pp. 131,168). 
 75. KIRCHOFF, Trans. Berlin Acad. 1861. 
 
 BALFOUR STEWART on the theory of Exchanges (Report 
 
 British Association, 1861). 
 
 77. T. WEDGWOOD, On the Production of Light and Heat by 
 
 different Bodies (Phil. Trans, vol. Ixxxii. p. 272). 
 
 78. TYKDALL, Proc R. S. vol. iv. p. 487, 
 
 81. GROVE, On the Decomposition of Water into its Constituent 
 Gases by Heat (Phil. Trans. 1847, p. 1).
 
 NOTES AND REFERENCES. 353 
 
 PACK 
 
 81. ROBINSON, On the Effect of Heat in lessening the Affinities 
 of the Elements of Water (Transactions of the Royal 
 Irish Academy, vol. xxi. p. 2). 
 
 83. GROVE, Water decomposed by Chlorine and Heat (Phil. 
 
 Trans. 1847, p. 20). 
 87. CARNOT, Reflexions sur la Puissance motrice du Feu, 
 
 Paris, 1824. 
 
 96. SEGUIN, Influence des Chemins de Fer, p. 378 et seq. 
 
 98. ROGERS, Consumption of Coal for Man power (Cosmos, vol. 
 ii. p. 5G). 
 
 102. MAYER and WATERSTON have suggested that solar heat may 
 
 arise from the mechanical action of meteoric stones falling 
 ~into the sun, and Mr. THOMPSON has written an elaborate 
 paper on the subject (Trans. Brit. Assoc. 1853). If a 
 number of gravitating bodies exist in the neighbourhood 
 of the sun, and form, as is conjectured, the zodiacal 
 light, it is difficult to conceive how comets as they 
 approach this region steer clear of such bodies, and are 
 not even de'flected from their orbits. 
 
 For Mr. THOMPSON'S various and valuable papers, see Phil. 
 Mag. passim. 
 
 103. POISSON, Comptes rendus, Paris, January 30, 1837. 
 
 106. DUFAYE, SYMMER, WATSON, and FRANKLIN, Theories of 
 
 Electric Fluid and Electric Fluids (Priestley's History of 
 Electricity, pp. 429-441). 
 
 107. GROTTHUS, Sur la Decomposition de 1'Eau et des Corps 
 
 qu'elle tient en dissolution a aide de 1'Electricite galva- 
 
 nique (Ann. de Chimie, torn. Iviii. p. 54). 
 FARADAY, On the Question whether Electrolytes conduct 
 
 without Decomposition (Proceedings of the Weekly 
 
 Meetings of the Royal Institution, 1855). 
 GROVE (Comptes rendus, Paris, 1839). 
 
 108. FARADAY, On Induction as an Action of contiguous Par- 
 
 ticles (Phil. Trans. 1838, p. 30). 
 
 109. MATTEUCCI, Plates of Mica polarised by Electricity (De la 
 
 Rive's Electricity, p. 140). 
 A A
 
 354 NOTES AND REFERENCES. 
 
 PAGE 
 
 109. GROVE, Electrolysis across Glass (Phil. Mag. Aug. 1860). 
 
 110. KABSTEN on Electrical Figures (Archiv. de 1'Elec. vols. ii. 
 
 iii. and iv.) 
 
 111. GROVE, Etching Electrical Figures and transferring them to 
 
 Collodion (Phil. Mag. January 1857). 
 
 113. FUSINIERI, Du Transport des Matieres ponderables qui 
 
 s'opere dans les De"charges electriques (Archives de 
 I'Electricite* ; Supplement a la Bibliotheque universelle 
 de Geneve, torn. iii. p. 597). 
 
 114. GROVE, On the Voltaic Arc (Report of Lecture at the 
 
 Royal Institution, Lit. Gaz. and Athenaeum, Feb. 7, 
 1845 ; Phil. Trans. 1847, p. 16). 
 
 116 to 122. GROVE, On the Electro-chemical Polarity of Gases 
 (Phil. Trans. 1852, p. 87). 
 
 121. FREMY and E. BECQUEREL, Oxygen changed to Ozone by the 
 Electric Spark (Ann. de Ch. et de Phys. 1852). This 
 subject and the nature of Ozone \vas first investigated by 
 Dr. Schiinbein. See also a paper by Mr. Brodie On the 
 Conditions of certain Elements at the Moment of Chemi- 
 cal Change (Phil. Trans. 1850). 
 
 123, 124. Molecular Changes in Electrised Metals (NAIRNE, 
 Phil. Trans. 1780, p. 334, and 1783, p. 223 ; GIIOVE, 
 Electrical Mag. vol. i. p. 120; PELTIER, Archives de 
 I'Electricite", vol. v. p. 182 ; FUSINIERI, id. p. 516). 
 
 125. WERTHEIM, Change in Elasticity of Metals by Electrisation 
 
 (Ann. de Ch. et de Phys. vol. xii. p. 623 ; Arch. Elec. 
 vol. iv. p. 490). 
 
 DUFOUR, Alteration in Tenacity of Metals by Electrisation 
 (Bibl. univ. de Geneve, Fev. 1855, p. 156). 
 
 126. MATTEUCCI, Conduction of Electricity by Crystals (Comptes 
 
 rendus de 1'Acad., Paris, March 5, 1855, p. 541). 
 128. E. BECQUEREL, Transmission of Electricity by heated Gases 
 
 (Ann. de Ch. et de Phys. vol. xxxix. p. 355). 
 GROVE, Proceedings of the Royal Inst. (1854, p. 361). 
 BECQUEREL, Divergence of Gold-Leaves in Vacuo (Traite 
 
 d'Electricite, vol. v. ; part ii. p. 55).
 
 NOTES AND REFERENCES. 355 
 
 PAGE 
 
 128. NEWTON, Thirty-first Query to the Optics. 
 
 130. GROVE, Particles of Metals and Metallic Oxides detached 
 
 in Liquids by Electricity (Elec. Mag. vol. i. p. 119). 
 
 131. MATTEUCCI, Relations of Electricity and Nervous Force 
 
 (Phil. Trans. 1845, p. 285, 1846, p. 497 ; Phenomenes 
 physiques des Corps vivants, p. 305 ; Lezioni di Fisica, 
 p. 360). 
 
 GALVANI VOLTA MARIANINJ et NOBILI on Physiological 
 Effects of Electricity (Ann. de Ch. et de Phys. vols. 
 23, 25, 29, 38, 40, 43, 44, 56). 
 
 133. BECQUEREL, Chemical Changes by Friction (Traite" de 
 1'Elec. vol. v. part 1, p. 16). 
 
 139. DE LA RIVE, Heat of the Voltaic Pile (Bibl. univ. vol. xiii. 
 
 p. 389). 
 
 DAVY, On the Properties of Electrified Bodies in their 
 relations to Conducting Powers and Temperature (Phil. 
 Trans, 1821, p. 428). 
 
 140. GROVE, On the Effect of surrounding Media on Voltaic 
 
 Ignition (Phil Trans. 1849, p. 49). 
 
 141. OERSTED, Experiences sur 1'Effet du Conflit electrique 
 
 sur 1' Aiguille aimant^e (Ann. de Ch. et de Phys. torn, 
 xiv. p. 417). 
 
 142. COLERIDGE, Table Talk, vol. i. p. 65. 
 
 143. LENZ and JACOBI, Pogg. Ann. vol. xvlii. p. 403 ; Bulletin 
 
 de 1'Acad. St. Petersburg, 1839; Harris, Magnetism, 
 
 part 2, p. 63. 
 DAVY, Decomposition of the fixed Alkalies (Phil. Trans. 
 
 1808, p. 1). 
 BECQUEREL Des Composes electro-chimiques (Trait de 
 
 1'Electricite, vol. iii. c. 13). 
 CROSSE, Transactions of the British Association, vol. v. 
 
 p. 47 ; Proceedings of the Electrical Society, p. 320. 
 
 144. MALUS, Polarisation of Light by Reflexion (Memoires 
 
 d'Arcueil, torn. ii. p. 143). 
 
 ARAGO, Circular Polarisation by Solids (Memoires de 
 1'Institut, 1811). 
 
 * AA2
 
 856 NOTES AND REFERENCES. 
 
 FAOB 
 
 145. BIOT, Circular Polarisation by Liquids (Me"moires de 1'In- 
 
 stitut, 1817). 
 
 146. NIEPCE and DAGUERRE, Historique et Description des Pro- 
 
 code's du Daguerreotype, Paris, 1839. 
 TALBOT, Photogenic Drawing and Calotype (Phil. Mag. 
 March 1839, and August 1841). 
 
 149. HERSCHEL, Chemical Action of the Solar Spectrum on 
 various Substances (Phil. Trans. 1840, p. 1, and 1842, 
 p. 181). 
 HUNT, Researches on Light, London, 1844. 
 
 153. GROVE, Other Forces produced by Light (Lit. Gaz. January 
 
 1844). 
 
 154. GROVE, Influence of Light on the Polarised Electrode (Phil. 
 
 Mag. December 1858). 
 SOMERVILLE (Mrs.), On the Magnetising Power of the more 
 
 Refrangible Solar Rays (Phil. Trans. 1826, p. 132). 
 MORICHINI'S experiments are given in Mrs. Somerville's 
 
 paper. 
 
 155. HERSCHEL, On the Absorption of Light in Coloured Media 
 
 viewed in connection with the Undulatory Theory (Phil. 
 
 Mag. December 1833). 
 TYNDALL, Proc. R. I., vol. iv. p. 491. 
 SEEBECK, Heat of Coloured Rays (Brewster's Optics, p. 90). 
 
 156. KNOBLAUCH (Ann. de Ch. vol. xxxvi. p. 124, and Pogg. 
 
 Ann. there referred to). 
 158. HERSCHEL, Epipolised Light (Phil. Trans, vol. cxxxv. 
 
 pp. 143, 147). 
 
 STOKES, Change in Refrangibility of Light (Phil. Trans, 
 vols. cxlii. cxliii.) 
 
 163. For the first enunciations of the Corpuscular and Undu- 
 
 latory Theories, see NEWTON'S Optics, HOOKE'S Micogra- 
 phia, and HUTGHENS' Tractatus de Lumine. See also 
 BREWSTER'S Optics, p. 138. 
 
 164. YOUNG, Lectures edited by Kelland, p. 359 et seq. ; Phil. 
 
 Trans. 1800, p. 126 ; HERSCHEL, Encyc. Metrop. art.
 
 NOTES AND REFERENCES. 357 
 
 PA.GK 
 
 Light, pp. 450 and 738 ; NEWTON'S Optics, p. 322 ; 
 WHEWELL'S Hist. Indue. Sc. vol. ii. p. 449 ; FOUCAULT, 
 Comptes rendus, Paris, 1850, p. 65 ; HARRISON, Phil. 
 Mag. November 1856 ; Camb. Phil. Trans. 
 
 167. SONDEAUSS, Refraction of Sound (Ann. de Ch. et de Phys. 
 vol. xxxv. p. 505) ; DOVE, Polarisation of Sound 
 (Cosmos, May 13, 1859). 
 
 176. PASTEUR, Rotation of Plane of Polarised Light by Solutions 
 of Hemihedral Crystals (Ann. de Ch. et de Phys. vol. 
 xxiv. p. 442). 
 
 179 to .183. WOLLASTON, Phil. Trans. 1822, p. 89 ; WHEWELL, 
 Phil, of the Induct. Sc. vol. i. p. 419 ; WILSON, Trans, 
 of the Roy. Soc. of Edin. vol. xvi. p. 79 ; Sir. TV. HER- 
 SCHEL, Phil. Trans. 1793, p. 201, and 1801, p. 300 ; 
 MORGAN, Phil. Trans, vol. Ixxv. p. 272 ; DAVY, Phil. 
 Trans. 1822, p. 64 ; Elements of Chemical Philosophy, 
 p. 97 ; GASSIOT, Phil. Trans. 1859, p. 157. 
 
 184. Diminishing .Periods of Comets (Herschel's Outlines of 
 Astronomy, p. 357). 
 
 187. Since writing the passage in the text, I find that STRUVE 
 has been led, from his astronomical researches, to the 
 conclusion that some light is lost in the interplanetary 
 spaces. He gives as an approximation one per cent, as 
 lost by the passage of light from a star of the first mag- 
 nitude, assuming a mean or average distance (Etudes 
 d' Astronomic SteUaire, 1847). 
 NEWTON, Thirtieth Query to the Optics. 
 
 190. FARAD AT, Evolution of Electricity from Magnetism (Phil. 
 Trans. 1832, p. 125). 
 
 193. FARADAY, Magnetic Condition of all Matter (Phil. Trans. 
 
 1846, p. 21 ; Phil. Mag. 1846, p. 249). 
 BECQUEREL, Ann. de Ch. et de Ph. torn, xxxvi. p. 337 ; 
 Comptes rendus, Paris, 1846, p. 147 ; and 1850, p. 201. 
 
 194. FARADAY, On the Magnetisation of Light (Phil. Trans. 
 
 1846, p. 1).
 
 358 NOTES AND REFERENCES. 
 
 PAGE 
 
 195. WARTMANN, Rotation of the Plane of Polarisation of Heat 
 
 by Magnetism (Journal de 1'Institut, No. 644). 
 PROVOSTATE and DESSAINES, Ann. de Ch. et de Phys.* 
 October 1849. 
 
 196. HUNT, Influence of Magnetism on Molecular Arrangement 
 
 (Phil. Mag. 1846, vol. xxviii. p. 1 ; Memoirs of the 
 Geological Society, vol. i. p. 433). 
 WARTMANN, Phil. Mag. 1847, vol. xxx. p. 263. 
 
 197. GROVE, Experiment on Molecular Motion of a Magnetic 
 
 Substance (Electrical Mag. 1845, vol. i. p. 601). 
 
 198. On the direct Production of Heat by Magnetism (Proceed- 
 
 ings of the Royal Society, 1849, p. 826). 
 
 After this paper was communicated and ordered to be 
 printed in the Philosophical Transactions, I found that I 
 had been anticipated by Mr. VAN BREDA, who communi- 
 cated, in 1845, a paper to the Institut on the subject ; his 
 paper appears in the Comptes rendus under an erroneous 
 title, which accounts for ita having been overlooked ; he 
 does not give thermometric measures of the heat he ob- 
 tained, nor did he produce heating effects by a permanent 
 steel magnet, or with other metals than iron. (Comptes 
 rendus, October 27, 1845). 
 
 See also an earlier experiment by Mr. JOULE (Phil. Mag. 
 1843), to which he called my attention after my paper was 
 read. 
 
 199. 203. The Experiments on the effects of Magnetism on the 
 
 Matter magnetised, are collected by Mr. DE LA RIVE in 
 his recently-published Treatise on Electricity, vol. i. 
 
 206. DAVY, Electricity defined as Chemical affinity acting on 
 
 Masses (Phil. Trans. 1826, p. 389). 
 
 VOLTA, Electricity excited by the mere Contact of conduct- 
 ing Substances (Phil. Trans. 1800, p. 403). 
 
 207. GROVE, Gold-Leaf Experiment (Comptes rendus, Paris, 
 
 1839, p. 567). 
 
 208. GROVE, Voltaic Action of Sulphur, Phosphorus, and Hy- 
 
 drocarbons (Phil. Trans. 1845, p. 351).
 
 NOTES AND REFERENCES. 359 
 
 PA6B 
 
 208. GROVE, New Voltaic Combination (Phil. Mag. vol. xiv. 
 p. 388 ; vol. xv. p. 287). 
 
 GROVE, Electricity of Blowpipe Flame (Proceedings of the 
 Royal Institution, February 1854), Phil. Mag. 
 
 211. D ALTON, New System of Chemistry, London, 1810. 
 
 212. I have here and elsewhere used whole numbers, as suffi- 
 
 ciently approximate for the argument, but without in- 
 tending to express any opinion as to the law of PROUT. 
 
 213. FARADAY, Definite Electrolysis (Phil. Trans. 1834, p. 77). 
 
 216. WOOD, Heat disengaged in Chemical Combinations (Phil. 
 
 Mag. 1852). 
 218. ANDREWS, Phil. Trans. 1844, p. 21. 
 
 HESS, PoggendorfTs Annalen, Bd. lii. p. 107. 
 220. FAVRE, Ann. de Ch. et de Phys. vols. 39, 40 ; Comptes 
 
 rendus, Paris, vol. 45, p. 56, and vol. 46, p. 337. 
 
 228. CATALYSIS by Platinum (DOBEREIMER, Ann. de Ch. et de 
 
 Phys. torn. xxiv. p. 93 ; DULONG and THEXARD, Ann. de 
 Ch. et de Phys. torn, xxiii. p. 440). 
 
 229. GROVE, Gas Voltaic Battery (Phil. Mag. February 1839, 
 
 and December 1842 ; Phil. Trans. 1843, p. 91). 
 
 231. MOSOTTI, Forces which regulate the Internal Constitution 
 
 of Bodies (Taylor's Scientific Memoirs, vol. i. p. 448). 
 
 232. PLUCKER, Repulsion of the Optic Axes of Crystals by the 
 
 Poles of a Magnet (Taylor's Scientific Memoirs, vol. v. 
 p. 353). 
 Magnetic Action of Cyanite (Lit. Gaz. 1849, p. 431). 
 
 233. MATTECCCI, Correlation of Electric Current and Nervous 
 
 Force (Phil. Trans. 1850, p. 287). 
 
 234. CARPENTER, On the Mutual Relations of the Vital and 
 
 Physical Forces (Phil. Trans. 1850, p. 751). 
 
 236. On Effort. See BROWN, Cause and Effect; HERSCHEL'S 
 
 Discourse ; and QUARTERLY REVIEW, June 1841. 
 
 237. HELMHOLTZ, Muller's Archives, 1845 ; MATTEUCCI, Comptes
 
 360 NOTES AND REFERENCES. 
 
 PAGE 
 
 rendus, Paris, 1856 ; BECLARD, Archives de Medecine, 
 1861. 
 
 256. DULONG and PETIT, Relation between Specific Heat and 
 
 Chemical Equivalents (Ann. de Ch. et de Phys. torn. x. 
 p. 395). 
 
 257. NEUMANN, PoggendorfTs Annalen, Bd. xxiii. p. 1. 
 AVOGADRO, Ann. de Ch. et de Phys. torn. Iv. p. 80. 
 
 NOTES AND REFERENCES 
 
 TO 
 
 CONTINUITY. 
 
 281. HERSCHEL, Sir J., Astronomical Observations at the Capo 
 
 of Good Hope, 1847. 
 
 KOSSE, Earl of, Observations on the Nebula? (Phil. Trans. 
 1850, p. 499). 
 
 282. OLMSTED, Silliman's Journal, July 1834, p. 138. 
 
 The first suggestion of a perspective vanishing point for 
 
 meteors seems to be due to Professor Thomson, of 
 
 Nashville. 
 HERSCHEL, ALEXANDER, Reports of the Meteor Committee 
 
 of the British Association. 
 SORBY, Idem, and Proc. R. S. June 16, 1864. 
 BRAYLEY, Idem, 1865, p. 140, and Proc. R. S. March 23, 
 
 1865. 
 
 283. LEVERRIER, Intra Mercurial Planets (Comptes rendus, 
 
 Paris, 1861, p. 1109). 
 
 284. DAUBRE"E, Comptes rendus, Paris, 1866 (Bulletin de la 
 
 Socie'te' Gfologique de France, Mars 1866.) 
 287. PLUCKER, Variation of Spectrum Lines with Temperature 
 (Phil. Trans. 1865, p. 6.)
 
 NOTES AND REFERENCES. 361 
 
 PJLGB 
 
 288. HUGGINS and MILLER, Spectra of Fixed Stars (Phil. Trans. 
 
 1864, p. 413). 
 
 Spectrum of Temporary Star (Proc. E. S., No. 84, 1866). 
 HUGGINS, Spectrum of Comet, 1866 (Proc. E. S., No. 80, 
 
 1866). 
 
 291. CHACORNAC, On the Moon (Comptes rendus, Paris, June 
 1866, p. 1406, &c.). 
 
 297. EUMFORD, Heat of Friction (Phil. Trans. 1798, p. 80). 
 DAVY, Idem, West of England Contributions, p. 18. 
 
 298. SABINE, Magnetism and Solar Spots (Proc. E, S. 1865, 
 
 p. 491). 
 
 299. AIRY, On Solar Magnetism (Phil. Trans. 1863, pp. 313- 
 
 646). 
 
 CHAMBERS, Idem, Phil. Trans. 1863, pp. 514-516. 
 MAYER, Friction of Tidal Wave (See his Papers collected 
 
 and translated by Youmans, New York, 1865). 
 
 300. DELAUNAY, Acceleration of Moon's Motion (Comptes rendus, 
 
 Paris, Dec. 1865, Jan. 1866). 
 AIRY, Idem, Notices E. Ast. Soc., April 13, 1866. 
 CARRINGTON, Observations on Spots on the Sun, 1863. 
 DE LA EUE, STEWART, and LOEWY, Idem 1865. 
 
 301. FA YE, On the Dynamic Theory of Solar Heat (Comptes 
 
 rendus, Paris, Oct. 1862, p. 564). 
 
 Constitution of Sun, Motion of Sun Spots, &c. (Comptes 
 rendus, Paris, Jan. 1866, &c). 
 
 302. STRUVE, Etudes d'Astronomie Stellaire, 1847. 
 
 The passage in the text is so brief as to be obscure (See 
 the idea elaborated, Corr. Phys. Forces, p. 187% 
 
 303. See Corr. Phys. Forces, p. 84. 
 
 304. BERTHELOT, Formate of Potash (Institut. 1864, p. 332). 
 TYNDALL, On Eadiant Heat (Phil. Mag., Nov. 1864 ; Phil. 
 
 Trans. 1866). 
 
 305. GRAHAM, Dialysis of Air (Phil. Trans. 1866, p. 399). 
 
 306. WILDE, Increase of Magneto-Electric Force (Proc. E. S., 
 
 April 1866, p. 107).
 
 362 NOTES AND REFERENCES. 
 
 PAGE 
 
 306. HOLTZ, New Electrical Machine (Pogg. Annalen, 1865, 
 pt. 1, p. 157). 
 
 308. CARPENTER, Food and Force ; Physiology, Treatise on. 
 BENCE JONES, Idem, Proc. R. L March 23, 1866. 
 PLATFAIR, Idem, Proc. R. I. April 28, 1865. 
 
 E. SMITH, Idem, Phil. Trans. 1861, p. 747. 
 FBANKLAND, Idem, Proc. R. I. 1866. 
 
 309. TRAUBE, Virchow's Archiv. vol. xxiii. p. 196, &c. 
 
 FICK and WISLICENUS, Idem, Phil. Mag. June 1866, Supple- 
 ment. 
 
 311. LAVOISIER, CEuvres, vol. ii. p. 640. 
 
 312. ANSTED, Intellectual Observer, Aug. 1864. 
 
 313. RAMSAY, Addresses to the Geological Society, 1863 and 
 
 1864. 
 
 314. HERSCHEL, Sir J., Geological Effects of Variation in Earth's 
 
 Orbit (Trans. Geol. Soc., 2nd series, vol. iii. p. 295 ; 
 Outlines of Astronomy, 1864, pp. 233-235). 
 CROLL, Idem, Phil. Mag., Aug. 1864, and Apr. 1866. 
 
 320. PASTEUR and POUCHET, On Spontaneous Generation 
 
 (Comptea rendus, Paris, 1863 to 1865 inclusive). 
 GUILD, Proc. R. S. 1865, p. 178. 
 
 326. CARPENTER, On Foraminifera (Phil. Trans. 1856, p. 227 ; 
 1860, p. 584). 
 
 328. II. BATES, Butterflies of South America (Trans. Linn. Soc. 
 vol. xxiii. p. 495). 
 
 330. WALLACE, Butterflies of Malay, &c. (Trans. Linn. Soc., vol. 
 
 xxv. p. 1). 
 
 WALSH, Proc. Entom. Soc. Philadelphia, 1864, p. 403. 
 
 331. FRITZ MtiLLER, Fur Darwin, Leipsig, 1864 (Annals and 
 
 Magazine of Natural History, 1865). 
 LUBBOCK, Diving Hymenoptera (Trans. Linn Soc. vol. xxiv. 
 
 p. 135). 
 LOGAN, Eozoon, Communication to the British Association 
 
 at Bath, 1864.
 
 NOTES AND REFERENCES. 363 
 
 A. DECANDOLLE, Variability in Oaks, &c. (Bibl. Univ. de 
 Geneve, Nov. 1862). 
 
 331. HOOKER, On Oaks (Trans. Linn. Soc. vol. xxiii. p. 381). 
 
 332. On Arctic Flora (Trans. Linn. Soc. vol. raJi- p. 251). 
 342. DARWIN, Origin cf Species through natural Selection, 
 
 1866, in which see also Dr. McDonnell's results. 
 HUXLEY, Address to the Geological Society, Feb. 21, 18G2. 
 LYELL, Antiquity of Man, 1863. 
 
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